diff --git a/src/Contracts/RetrievesStockSkus.php b/src/Contracts/RetrievesStockSkus.php
index 6a3ce91..35386d2 100644
--- a/src/Contracts/RetrievesStockSkus.php
+++ b/src/Contracts/RetrievesStockSkus.php
@@ -11,5 +11,5 @@ interface RetrievesStockSkus
     public function retrieveAll(): Enumerable;
 
     /** @return Enumerable<int, string> */
-    public function retrieveUpdated(Carbon $from = null): Enumerable;
+    public function retrieveUpdated(?Carbon $from = null): Enumerable;
 }
diff --git a/src/Exceptions/UpdateException.php b/src/Exceptions/UpdateException.php
index 69367f5..ea08732 100644
--- a/src/Exceptions/UpdateException.php
+++ b/src/Exceptions/UpdateException.php
@@ -7,7 +7,7 @@
 
 class UpdateException extends Exception
 {
-    public function __construct(string $sku, string $message, public array $payload = [], Throwable $previous = null)
+    public function __construct(string $sku, string $message, public array $payload = [], ?Throwable $previous = null)
     {
         parent::__construct("Failed to update $sku: $message", 0, $previous);
     }
diff --git a/src/Retriever/DummySkuRetriever.php b/src/Retriever/DummySkuRetriever.php
index 061d14e..e2b04a5 100644
--- a/src/Retriever/DummySkuRetriever.php
+++ b/src/Retriever/DummySkuRetriever.php
@@ -13,7 +13,7 @@ public function retrieveAll(): Enumerable
         return collect(['::sku_1::', '::sku_2::']);
     }
 
-    public function retrieveUpdated(Carbon $from = null): Enumerable
+    public function retrieveUpdated(?Carbon $from = null): Enumerable
     {
         return collect(['::sku_1::']);
     }