diff --git a/php/lib/shop_cart_manager.php b/php/lib/shop_cart_manager.php
index 251b7b1e..4b24f30b 100644
--- a/php/lib/shop_cart_manager.php
+++ b/php/lib/shop_cart_manager.php
@@ -44,7 +44,7 @@ public function commit_string()
 	{
 	   return '('
 	   			. $this->_cart_id 		. ','
-	   			. $this->_order_item_id . ','
+	   			. ( $this->_order_item_id != 'undefined' ? $this->_order_item_id : 'null' ) . ','
 	   			. $this->_product_id 	. ','
 	   			. $this->_quantity 		. ','
 	   			. $this->_iva_percent	. ','