diff --git a/integration-tests/src/helpers.rs b/integration-tests/src/helpers.rs
index 9be15c4..5495e51 100644
--- a/integration-tests/src/helpers.rs
+++ b/integration-tests/src/helpers.rs
@@ -1,7 +1,7 @@
 use serde_json::json;
 use near_workspaces::{types::{NearToken, AccountDetails}, Account, Contract};
 
-pub const DEFAULT_DEPOSIT: u128 = 10000000000000000000000 as u128;
+pub const DEFAULT_DEPOSIT: u128 = 10000000000000000000000;
 pub const ONE_YOCTO_NEAR: NearToken = NearToken::from_yoctonear(1);
 
 pub async fn mint_nft(
@@ -70,7 +70,7 @@ pub async fn place_nft_for_sale(
     market_contract: &Contract,
     nft_contract: &Contract,
     token_id: &str,
-    approval_id: u128,
+    approval_id: u32,
     price: &NearToken,
 ) -> Result<(), Box<dyn std::error::Error>> {
     let request_payload = json!({
diff --git a/market-contract/src/external.rs b/market-contract/src/external.rs
index 7700642..91540ff 100644
--- a/market-contract/src/external.rs
+++ b/market-contract/src/external.rs
@@ -10,7 +10,7 @@ trait ExtContract {
         &mut self,
         receiver_id: AccountId, //purchaser (person to transfer the NFT to)
         token_id: TokenId, //token ID to transfer
-        approval_id: u64, //market contract's approval ID in order to transfer the token on behalf of the owner
+        approval_id: u32, //market contract's approval ID in order to transfer the token on behalf of the owner
         memo: String, //memo (to include some context)
         /*
             the price that the token was purchased for. This will be used in conjunction with the royalty percentages
@@ -25,6 +25,6 @@ trait ExtContract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
     );
 }
\ No newline at end of file
diff --git a/market-contract/src/nft_callbacks.rs b/market-contract/src/nft_callbacks.rs
index 02ab75e..1d4bd68 100644
--- a/market-contract/src/nft_callbacks.rs
+++ b/market-contract/src/nft_callbacks.rs
@@ -12,7 +12,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -24,7 +24,7 @@ impl NonFungibleTokenApprovalsReceiver for Contract {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     ) {
         /*
diff --git a/market-contract/src/sale.rs b/market-contract/src/sale.rs
index 051fe0e..e435fe4 100644
--- a/market-contract/src/sale.rs
+++ b/market-contract/src/sale.rs
@@ -10,7 +10,7 @@ pub struct Sale {
     //owner of the sale
     pub owner_id: AccountId,
     //market contract's approval ID to transfer the token on behalf of the owner
-    pub approval_id: u64,
+    pub approval_id: u32,
     //nft contract where the token was minted
     pub nft_contract_id: String,
     //actual token ID for sale
@@ -35,7 +35,7 @@ impl Contract {
       &mut self,
       nft_contract_id: AccountId,
       token_id: TokenId,
-      approval_id: u64,
+      approval_id: u32,
       sale_conditions: SalePriceInYoctoNear,
     ) {
         let owner_id = env::predecessor_account_id();
@@ -267,7 +267,7 @@ impl Contract {
         owner_id: AccountId,
         nft_contract_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         sale_conditions: SalePriceInYoctoNear,
         #[callback_result] nft_token_result: Result<JsonToken, PromiseError>,
         #[callback_result] nft_is_approved_result: Result<bool, PromiseError>,
diff --git a/market-contract/src/sale_views.rs b/market-contract/src/sale_views.rs
index 9dcf7b4..b1789c2 100644
--- a/market-contract/src/sale_views.rs
+++ b/market-contract/src/sale_views.rs
@@ -32,8 +32,8 @@ impl Contract {
     pub fn get_sales_by_owner_id(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<Sale> {
         //get the set of token IDs for sale for the given account ID
         let by_owner_id = self.by_owner_id.get(&account_id);
@@ -48,7 +48,7 @@ impl Contract {
         let keys = sales.as_vector();
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
         
         //iterate through the keys vector
         keys.iter()
@@ -82,8 +82,8 @@ impl Contract {
     pub fn get_sales_by_nft_contract_id(
         &self,
         nft_contract_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<Sale> {
         //get the set of token IDs for sale for the given contract ID
         let by_nft_contract_id = self.by_nft_contract_id.get(&nft_contract_id);
@@ -99,7 +99,7 @@ impl Contract {
         let keys = sales.as_vector();
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
         
         //iterate through the keys vector
         keys.iter()
diff --git a/nft-contract-approval/src/approval.rs b/nft-contract-approval/src/approval.rs
index 3dda71a..6ea2317 100644
--- a/nft-contract-approval/src/approval.rs
+++ b/nft-contract-approval/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -54,7 +54,7 @@ impl NonFungibleTokenCore for Contract {
         );
 
         //get the next approval ID if we need a new approval
-        let approval_id: u64 = token.next_approval_id;
+        let approval_id: u32 = token.next_approval_id;
 
         //check if the account has been approved already for this token
         let is_new_approval = token
@@ -99,7 +99,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         //get the token object from the token_id
         let token = self.tokens_by_id.get(&token_id).expect("No token");
diff --git a/nft-contract-approval/src/enumeration.rs b/nft-contract-approval/src/enumeration.rs
index 5926561..49ebc6f 100644
--- a/nft-contract-approval/src/enumeration.rs
+++ b/nft-contract-approval/src/enumeration.rs
@@ -9,9 +9,9 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each token using an iterator
         self.token_metadata_by_id.keys()
@@ -46,8 +46,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         //get the set of tokens for the passed in owner
         let tokens_for_owner_set = self.tokens_per_owner.get(&account_id);
@@ -60,7 +60,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the keys vector
         tokens.iter()
diff --git a/nft-contract-approval/src/internal.rs b/nft-contract-approval/src/internal.rs
index d1695d8..54ce98c 100644
--- a/nft-contract-approval/src/internal.rs
+++ b/nft-contract-approval/src/internal.rs
@@ -22,7 +22,7 @@ pub(crate) fn refund_approved_account_ids_iter<'a, I>(
 //refund a map of approved account IDs and send the funds to the passed in account ID
 pub(crate) fn refund_approved_account_ids(
     account_id: AccountId,
-    approved_account_ids: &HashMap<AccountId, u64>,
+    approved_account_ids: &HashMap<AccountId, u32>,
 ) -> Promise {
     //call the refund_approved_account_ids_iter with the approved account IDs as keys
     refund_approved_account_ids_iter(account_id, approved_account_ids.keys())
@@ -134,7 +134,7 @@ impl Contract {
         receiver_id: &AccountId,
         token_id: &TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) -> Token {
         //get the token object by passing in the token_id
diff --git a/nft-contract-approval/src/metadata.rs b/nft-contract-approval/src/metadata.rs
index 1c9ab53..a582828 100644
--- a/nft-contract-approval/src/metadata.rs
+++ b/nft-contract-approval/src/metadata.rs
@@ -44,9 +44,9 @@ pub struct Token {
     //owner of the token
     pub owner_id: AccountId,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
     //the next approval ID to give out. 
-    pub next_approval_id: u64,
+    pub next_approval_id: u32,
 }
 
 //The Json token is what will be returned from view calls. 
@@ -60,7 +60,7 @@ pub struct JsonToken {
     //token metadata
     pub metadata: TokenMetadata,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
 }
 
 pub trait NonFungibleTokenMetadata {
diff --git a/nft-contract-approval/src/nft_core.rs b/nft-contract-approval/src/nft_core.rs
index 1be044e..6f04739 100644
--- a/nft-contract-approval/src/nft_core.rs
+++ b/nft-contract-approval/src/nft_core.rs
@@ -11,7 +11,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     );
 
@@ -22,7 +22,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool>;
@@ -59,7 +59,7 @@ trait NonFungibleTokenResolver {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool;
@@ -74,7 +74,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) {
         //assert that the user attached exactly 1 yoctoNEAR. This is for security and so that the user will be redirected to the NEAR wallet. 
@@ -105,7 +105,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool> {
@@ -189,7 +189,7 @@ impl NonFungibleTokenResolver for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool {
diff --git a/nft-contract-approval/src/royalty.rs b/nft-contract-approval/src/royalty.rs
index 7646f9e..0ee4c7a 100644
--- a/nft-contract-approval/src/royalty.rs
+++ b/nft-contract-approval/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -33,7 +33,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-contract-basic/src/approval.rs b/nft-contract-basic/src/approval.rs
index 9d72d6c..8e55739 100644
--- a/nft-contract-basic/src/approval.rs
+++ b/nft-contract-basic/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -47,7 +47,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         /*
             FILL THIS IN
diff --git a/nft-contract-basic/src/enumeration.rs b/nft-contract-basic/src/enumeration.rs
index 5926561..49ebc6f 100644
--- a/nft-contract-basic/src/enumeration.rs
+++ b/nft-contract-basic/src/enumeration.rs
@@ -9,9 +9,9 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each token using an iterator
         self.token_metadata_by_id.keys()
@@ -46,8 +46,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         //get the set of tokens for the passed in owner
         let tokens_for_owner_set = self.tokens_per_owner.get(&account_id);
@@ -60,7 +60,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the keys vector
         tokens.iter()
diff --git a/nft-contract-basic/src/royalty.rs b/nft-contract-basic/src/royalty.rs
index 7646f9e..0ee4c7a 100644
--- a/nft-contract-basic/src/royalty.rs
+++ b/nft-contract-basic/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -33,7 +33,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-contract-events/src/approval.rs b/nft-contract-events/src/approval.rs
index 9d72d6c..8e55739 100644
--- a/nft-contract-events/src/approval.rs
+++ b/nft-contract-events/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -47,7 +47,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         /*
             FILL THIS IN
diff --git a/nft-contract-events/src/enumeration.rs b/nft-contract-events/src/enumeration.rs
index 5926561..49ebc6f 100644
--- a/nft-contract-events/src/enumeration.rs
+++ b/nft-contract-events/src/enumeration.rs
@@ -9,9 +9,9 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each token using an iterator
         self.token_metadata_by_id.keys()
@@ -46,8 +46,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         //get the set of tokens for the passed in owner
         let tokens_for_owner_set = self.tokens_per_owner.get(&account_id);
@@ -60,7 +60,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the keys vector
         tokens.iter()
diff --git a/nft-contract-events/src/royalty.rs b/nft-contract-events/src/royalty.rs
index 7646f9e..0ee4c7a 100644
--- a/nft-contract-events/src/royalty.rs
+++ b/nft-contract-events/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -33,7 +33,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-contract-royalty/src/approval.rs b/nft-contract-royalty/src/approval.rs
index 3dda71a..6ea2317 100644
--- a/nft-contract-royalty/src/approval.rs
+++ b/nft-contract-royalty/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -54,7 +54,7 @@ impl NonFungibleTokenCore for Contract {
         );
 
         //get the next approval ID if we need a new approval
-        let approval_id: u64 = token.next_approval_id;
+        let approval_id: u32 = token.next_approval_id;
 
         //check if the account has been approved already for this token
         let is_new_approval = token
@@ -99,7 +99,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         //get the token object from the token_id
         let token = self.tokens_by_id.get(&token_id).expect("No token");
diff --git a/nft-contract-royalty/src/enumeration.rs b/nft-contract-royalty/src/enumeration.rs
index 5926561..49ebc6f 100644
--- a/nft-contract-royalty/src/enumeration.rs
+++ b/nft-contract-royalty/src/enumeration.rs
@@ -9,9 +9,9 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each token using an iterator
         self.token_metadata_by_id.keys()
@@ -46,8 +46,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         //get the set of tokens for the passed in owner
         let tokens_for_owner_set = self.tokens_per_owner.get(&account_id);
@@ -60,7 +60,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the keys vector
         tokens.iter()
diff --git a/nft-contract-royalty/src/internal.rs b/nft-contract-royalty/src/internal.rs
index 0f3895b..05e3aa2 100644
--- a/nft-contract-royalty/src/internal.rs
+++ b/nft-contract-royalty/src/internal.rs
@@ -27,7 +27,7 @@ pub(crate) fn refund_approved_account_ids_iter<'a, I>(
 //refund a map of approved account IDs and send the funds to the passed in account ID
 pub(crate) fn refund_approved_account_ids(
     account_id: AccountId,
-    approved_account_ids: &HashMap<AccountId, u64>,
+    approved_account_ids: &HashMap<AccountId, u32>,
 ) -> Promise {
     //call the refund_approved_account_ids_iter with the approved account IDs as keys
     refund_approved_account_ids_iter(account_id, approved_account_ids.keys())
@@ -139,7 +139,7 @@ impl Contract {
         receiver_id: &AccountId,
         token_id: &TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) -> Token {
         //get the token object by passing in the token_id
diff --git a/nft-contract-royalty/src/metadata.rs b/nft-contract-royalty/src/metadata.rs
index 3153b89..7d1c536 100644
--- a/nft-contract-royalty/src/metadata.rs
+++ b/nft-contract-royalty/src/metadata.rs
@@ -44,9 +44,9 @@ pub struct Token {
     //owner of the token
     pub owner_id: AccountId,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
     //the next approval ID to give out. 
-    pub next_approval_id: u64,
+    pub next_approval_id: u32,
     //keep track of the royalty percentages for the token in a hash map
     pub royalty: HashMap<AccountId, u32>,
 }
@@ -62,7 +62,7 @@ pub struct JsonToken {
     //token metadata
     pub metadata: TokenMetadata,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
     //keep track of the royalty percentages for the token in a hash map
     pub royalty: HashMap<AccountId, u32>,
 }
diff --git a/nft-contract-royalty/src/nft_core.rs b/nft-contract-royalty/src/nft_core.rs
index 61f4898..5286a97 100644
--- a/nft-contract-royalty/src/nft_core.rs
+++ b/nft-contract-royalty/src/nft_core.rs
@@ -11,7 +11,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     );
 
@@ -22,7 +22,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool>;
@@ -59,7 +59,7 @@ trait NonFungibleTokenResolver {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool;
@@ -74,7 +74,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) {
         //assert that the user attached exactly 1 yoctoNEAR. This is for security and so that the user will be redirected to the NEAR wallet. 
@@ -105,7 +105,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool> {
@@ -190,7 +190,7 @@ impl NonFungibleTokenResolver for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool {
diff --git a/nft-contract-royalty/src/royalty.rs b/nft-contract-royalty/src/royalty.rs
index 6e6cbb8..a1d6be9 100644
--- a/nft-contract-royalty/src/royalty.rs
+++ b/nft-contract-royalty/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -72,7 +72,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-contract-skeleton/src/approval.rs b/nft-contract-skeleton/src/approval.rs
index 9d72d6c..8e55739 100644
--- a/nft-contract-skeleton/src/approval.rs
+++ b/nft-contract-skeleton/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -47,7 +47,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         /*
             FILL THIS IN
diff --git a/nft-contract-skeleton/src/enumeration.rs b/nft-contract-skeleton/src/enumeration.rs
index d80f47a..c41d21b 100644
--- a/nft-contract-skeleton/src/enumeration.rs
+++ b/nft-contract-skeleton/src/enumeration.rs
@@ -11,7 +11,7 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         /*
             FILL THIS IN
         */
@@ -33,8 +33,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         /*
             FILL THIS IN
diff --git a/nft-contract-skeleton/src/royalty.rs b/nft-contract-skeleton/src/royalty.rs
index b71f42e..3eeea83 100644
--- a/nft-contract-skeleton/src/royalty.rs
+++ b/nft-contract-skeleton/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -32,7 +32,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-series/src/approval.rs b/nft-series/src/approval.rs
index 6755a53..4791558 100644
--- a/nft-series/src/approval.rs
+++ b/nft-series/src/approval.rs
@@ -10,7 +10,7 @@ pub trait NonFungibleTokenCore {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool;
 
     //revoke a specific account from transferring the token on your behalf
@@ -27,7 +27,7 @@ trait NonFungibleTokenApprovalsReceiver {
         &mut self,
         token_id: TokenId,
         owner_id: AccountId,
-        approval_id: u64,
+        approval_id: u32,
         msg: String,
     );
 }
@@ -54,7 +54,7 @@ impl NonFungibleTokenCore for Contract {
         );
 
         //get the next approval ID if we need a new approval
-        let approval_id: u64 = token.next_approval_id;
+        let approval_id: u32 = token.next_approval_id;
 
         //check if the account has been approved already for this token
         let is_new_approval = token
@@ -95,7 +95,7 @@ impl NonFungibleTokenCore for Contract {
         &self,
         token_id: TokenId,
         approved_account_id: AccountId,
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
     ) -> bool {
         //get the token object from the token_id
         let token = self.tokens_by_id.get(&token_id).expect("No token");
diff --git a/nft-series/src/enumeration.rs b/nft-series/src/enumeration.rs
index 2dd505b..f04c00e 100644
--- a/nft-series/src/enumeration.rs
+++ b/nft-series/src/enumeration.rs
@@ -25,9 +25,9 @@ impl Contract {
     }
 
     //Query for nft tokens on the contract regardless of the owner using pagination
-    pub fn nft_tokens(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonToken> {
+    pub fn nft_tokens(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonToken> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each token using an iterator
         self.tokens_by_id
@@ -60,8 +60,8 @@ impl Contract {
     pub fn nft_tokens_for_owner(
         &self,
         account_id: AccountId,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         //get the set of tokens for the passed in owner
         let tokens_for_owner_set = self.tokens_per_owner.get(&account_id);
@@ -74,7 +74,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the keys vector
         tokens
@@ -95,9 +95,9 @@ impl Contract {
     }
 
     // Paginate through all the series on the contract and return the a vector of JsonSeries
-    pub fn get_series(&self, from_index: Option<u128>, limit: Option<u64>) -> Vec<JsonSeries> {
+    pub fn get_series(&self, from_index: Option<U128>, limit: Option<u32>) -> Vec<JsonSeries> {
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through each series using an iterator
         self.series_by_id
@@ -107,19 +107,19 @@ impl Contract {
             //take the first "limit" elements in the vector. If we didn't specify a limit, use 50
             .take(limit.unwrap_or(50) as usize)
             //we'll map the series IDs which are strings into Json Series
-            .map(|series_id| self.get_series_details(series_id.clone()).unwrap())
+            .map(|series_id| self.get_series_details(U64(series_id.clone())).unwrap())
             //since we turned the keys into an iterator, we need to turn it back into a vector to return
             .collect()
     }
 
     // get info for a specific series
-    pub fn get_series_details(&self, id: u64) -> Option<JsonSeries> {
+    pub fn get_series_details(&self, id: U64) -> Option<JsonSeries> {
         //get the series from the map
-        let series = self.series_by_id.get(&id);
+        let series = self.series_by_id.get(&id.0);
         //if there is some series, we'll return the series
         if let Some(series) = series {
             Some(JsonSeries {
-                series_id: id,
+                series_id: id.0,
                 metadata: series.metadata,
                 royalty: series.royalty,
                 owner_id: series.owner_id,
@@ -131,9 +131,9 @@ impl Contract {
     }
 
     //get the total supply of NFTs on a current series
-    pub fn nft_supply_for_series(&self, id: u64) -> U64 {
+    pub fn nft_supply_for_series(&self, id: U64) -> U64 {
         //get the series
-        let series = self.series_by_id.get(&id);
+        let series = self.series_by_id.get(&id.0);
 
         //if there is some series, get the length of the tokens. Otherwise return -
         if let Some(series) = series {
@@ -146,12 +146,12 @@ impl Contract {
     /// Paginate through NFTs within a given series 
     pub fn nft_tokens_for_series(
         &self,
-        id: u64,
-        from_index: Option<u128>,
-        limit: Option<u64>,
+        id: U64,
+        from_index: Option<U128>,
+        limit: Option<u32>,
     ) -> Vec<JsonToken> {
         // Get the series and its tokens
-        let series = self.series_by_id.get(&id);
+        let series = self.series_by_id.get(&id.0);
         let tokens = if let Some(series) = series {
             series.tokens
         } else {
@@ -159,7 +159,7 @@ impl Contract {
         };
 
         //where to start pagination - if we have a from_index, we'll use that - otherwise start from 0 index
-        let start = from_index.unwrap_or(0);
+        let start = u128::from(from_index.unwrap_or(U128(0)));
 
         //iterate through the tokens
         tokens
diff --git a/nft-series/src/internal.rs b/nft-series/src/internal.rs
index 1e9f959..534db42 100644
--- a/nft-series/src/internal.rs
+++ b/nft-series/src/internal.rs
@@ -31,7 +31,7 @@ pub(crate) fn refund_approved_account_ids_iter<'a, I>(
 //refund a map of approved account IDs and send the funds to the passed in account ID
 pub(crate) fn refund_approved_account_ids(
     account_id: AccountId,
-    approved_account_ids: &HashMap<AccountId, u64>,
+    approved_account_ids: &HashMap<AccountId, u32>,
 ) -> Promise {
     //call the refund_approved_account_ids_iter with the approved account IDs as keys
     refund_approved_account_ids_iter(account_id, approved_account_ids.keys())
@@ -172,7 +172,7 @@ impl Contract {
         receiver_id: &AccountId,
         token_id: &TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) -> Token {
         //get the token object by passing in the token_id
diff --git a/nft-series/src/metadata.rs b/nft-series/src/metadata.rs
index 4f2f025..d2cfa5e 100644
--- a/nft-series/src/metadata.rs
+++ b/nft-series/src/metadata.rs
@@ -46,9 +46,9 @@ pub struct Token {
     //owner of the token
     pub owner_id: AccountId,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
     //the next approval ID to give out.
-    pub next_approval_id: u64,
+    pub next_approval_id: u32,
 }
 
 //The Json token is what will be returned from view calls.
@@ -64,7 +64,7 @@ pub struct JsonToken {
     //token metadata
     pub metadata: TokenMetadata,
     //list of approved account IDs that have access to transfer the token. This maps an account ID to an approval ID
-    pub approved_account_ids: HashMap<AccountId, u64>,
+    pub approved_account_ids: HashMap<AccountId, u32>,
     //keep track of the royalty percentages for the token in a hash map
     pub royalty: Option<HashMap<AccountId, u32>>,
 }
diff --git a/nft-series/src/nft_core.rs b/nft-series/src/nft_core.rs
index 6166edf..9f3d3a1 100644
--- a/nft-series/src/nft_core.rs
+++ b/nft-series/src/nft_core.rs
@@ -11,7 +11,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     );
 
@@ -22,7 +22,7 @@ pub trait NonFungibleTokenCore {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool>;
@@ -59,7 +59,7 @@ trait NonFungibleTokenResolver {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool;
@@ -74,7 +74,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
     ) {
         //assert that the user attached exactly 1 yoctoNEAR. This is for security and so that the user will be redirected to the NEAR wallet.
@@ -100,7 +100,7 @@ impl NonFungibleTokenCore for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce an approval ID so that people with that approval ID can transfer the token
-        approval_id: Option<u64>,
+        approval_id: Option<u32>,
         memo: Option<String>,
         msg: String,
     ) -> PromiseOrValue<bool> {
@@ -205,7 +205,7 @@ impl NonFungibleTokenResolver for Contract {
         receiver_id: AccountId,
         token_id: TokenId,
         //we introduce the approval map so we can keep track of what the approvals were before the transfer
-        approved_account_ids: HashMap<AccountId, u64>,
+        approved_account_ids: HashMap<AccountId, u32>,
         //we introduce a memo for logging the transfer event
         memo: Option<String>,
     ) -> bool {
diff --git a/nft-series/src/royalty.rs b/nft-series/src/royalty.rs
index 7e3ab35..25de1a5 100644
--- a/nft-series/src/royalty.rs
+++ b/nft-series/src/royalty.rs
@@ -9,7 +9,7 @@ pub trait NonFungibleTokenCore {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
@@ -87,7 +87,7 @@ impl NonFungibleTokenCore for Contract {
         &mut self,
         receiver_id: AccountId,
         token_id: TokenId,
-        approval_id: u64,
+        approval_id: u32,
         memo: Option<String>,
         balance: U128,
         max_len_payout: u32,
diff --git a/nft-series/src/series.rs b/nft-series/src/series.rs
index 72b7f9b..a72858b 100644
--- a/nft-series/src/series.rs
+++ b/nft-series/src/series.rs
@@ -10,7 +10,7 @@ impl Contract {
     #[payable]
     pub fn create_series(
         &mut self,
-        id: u64,
+        id: U64,
         metadata: TokenMetadata,
         royalty: Option<HashMap<AccountId, u32>>,
         price: Option<U128>
@@ -29,7 +29,7 @@ impl Contract {
         require!(
             self.series_by_id
                 .insert(
-                    &id,
+                    &id.0,
                     &Series {
                         metadata,
                         royalty,
@@ -37,7 +37,7 @@ impl Contract {
                             // We get a new unique prefix for the collection
                             account_id_hash: hash_account_id(&format!(
                                 "{}{}",
-                                id, caller
+                                id.0, caller
                             )),
                         }),
                         owner_id: caller,
@@ -58,12 +58,12 @@ impl Contract {
     /// Mint a new NFT that is part of a series. The caller must be an approved minter.
     /// The series ID must exist and if the metadata specifies a copy limit, you cannot exceed it.
     #[payable]
-    pub fn nft_mint(&mut self, id: u64, receiver_id: AccountId) {
+    pub fn nft_mint(&mut self, id: U64, receiver_id: AccountId) {
         // Measure the initial storage being used on the contract
         let initial_storage_usage = env::storage_usage();
 
         // Get the series and how many tokens currently exist (edition number = cur_len + 1)
-        let mut series = self.series_by_id.get(&id).expect("Not a series");
+        let mut series = self.series_by_id.get(&id.0).expect("Not a series");
         
         // Check if the series has a price per token. If it does, ensure the caller has attached at least that amount
         let mut price_per_token = NearToken::from_yoctonear(0); 
@@ -90,14 +90,14 @@ impl Contract {
         }
 
         // The token ID is stored internally as `${series_id}:${edition}`
-        let token_id = format!("{}:{}", id, cur_len + 1);
+        let token_id = format!("{}:{}", id.0, cur_len + 1);
         series.tokens.insert(&token_id);
-        self.series_by_id.insert(&id, &series);
+        self.series_by_id.insert(&id.0, &series);
 
         //specify the token struct that contains the owner ID
         let token = Token {
             // Series ID that the token belongs to
-            series_id: id,
+            series_id: id.0,
             //set the owner ID equal to the receiver ID passed into the function
             owner_id: receiver_id,
             //we set the approved account IDs to the default value (an empty map)