diff --git a/src/brc20/error.rs b/src/brc20/error.rs
index 04c21b9b10..a7d0aec516 100644
--- a/src/brc20/error.rs
+++ b/src/brc20/error.rs
@@ -81,6 +81,11 @@ pub enum BRC20Error {
 
   #[error("transferable owner not match {0}")]
   TransferableOwnerNotMatch(InscriptionId),
+
+  /// an InternalError is an error that happens exceed our expect
+  /// and should not happen under normal circumstances
+  #[error("internal error: {0}")]
+  InternalError(String),
 }
 
 impl<L: LedgerRead> From<BRC20Error> for Error<L> {
diff --git a/src/brc20/num.rs b/src/brc20/num.rs
index 50bdeed59b..24c81d197a 100644
--- a/src/brc20/num.rs
+++ b/src/brc20/num.rs
@@ -71,7 +71,10 @@ impl Num {
       self
         .0
         .to_bigint()
-        .unwrap()
+        .ok_or(BRC20Error::InternalError(format!(
+          "convert {} to bigint failed",
+          self.0
+        )))?
         .to_u128()
         .ok_or(BRC20Error::Overflow {
           op: String::from("to_u128"),