diff --git a/src/productList.php b/src/productList.php
index f925b4c..f081aff 100644
--- a/src/productList.php
+++ b/src/productList.php
@@ -117,6 +117,11 @@ public function __set(string $key, $value): self
         if ($value['quantity'] < 0) {
             throw new \Exception('The quantity of the cart must not be negative.');
         }
+
+        if ($key !== $value['id']) {
+            $key = $value['id'];
+        }
+
         $this->data[strtolower($key)] = $value;
 
         if ($this->getTax() < 0) {