diff --git a/mod/assign/locallib.php b/mod/assign/locallib.php
index 142039f827f..81283e3c9f5 100644
--- a/mod/assign/locallib.php
+++ b/mod/assign/locallib.php
@@ -3143,7 +3143,7 @@ class assign {
             return false;
         }
         $assign = clone $this->get_instance();
-        $assign->cmidnumber = $this->get_course_module()->id;
+        $assign->cmidnumber = $this->get_course_module()->idnumber;
 
         return assign_grade_item_update($assign, $gradebookgrade);
     }