diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8cbac6da30..10fae83725 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Fixed a bug where Edit Product pages would allow duplication for users that didn’t have permission to duplicate the product. ([#3819](https://github.com/craftcms/commerce/issues/3819))
+- Fixed a PHP error that could occur when updating a cart. ([#3842](https://github.com/craftcms/commerce/issues/3842))
 - Fixed a PHP error that could occur when adding an invalid address to a cart. ([#3848](https://github.com/craftcms/commerce/issues/3848))
 
 ## 4.7.2 - 2024-12-18
diff --git a/src/controllers/CartController.php b/src/controllers/CartController.php
index 7b57940d37..e2e93f0e85 100644
--- a/src/controllers/CartController.php
+++ b/src/controllers/CartController.php
@@ -138,6 +138,7 @@ public function actionUpdateCart(): ?Response
 
         // When we are about to update the cart, we consider it a real cart at this point, and want to actually create it in the DB.
         if ($this->_cart->id === null) {
+            // Make sure we have a fully saved cart before attempting any mutations.
             $this->_cart = $this->_getCart(true);
         }