diff --git a/Store/StoreItemsView.php b/Store/StoreItemsView.php
index 1b7da7a9..680d025d 100644
--- a/Store/StoreItemsView.php
+++ b/Store/StoreItemsView.php
@@ -18,12 +18,15 @@ class StoreItemsView extends SwatControl
      */
     protected $product;
 
+    /**
+     * The page source for this view's form action.
+     */
+    protected ?string $source = null;
+
     protected $default_quantity;
 
     protected $has_description = false;
 
-    protected string $source;
-
     /**
      * @var bool
      */