diff --git a/code/extensions/CMSMainEditFormExtension.php b/code/extensions/CMSMainEditFormExtension.php
index 9ed6777..84f26af 100644
--- a/code/extensions/CMSMainEditFormExtension.php
+++ b/code/extensions/CMSMainEditFormExtension.php
@@ -48,11 +48,7 @@ public function updateEditForm($form)
                     // Define Form Action (allows ViewerGroups field to work in asset EditForm):
                     
                     $dummy->setFormAction(
-                        sprintf(
-                            '%s/field/File/item/%d/ItemEditForm',
-                            $form->FormAction(),
-                            $record->ID
-                        )
+                        Controller::join_links($form->FormAction(), 'field/File/item', $record->ID, 'ItemEditForm')
                     );
                     
                     // Update Field Object: