diff --git a/mod/assignment/lib.php b/mod/assignment/lib.php
index 6c8dbba7a4e..df6da617e53 100644
--- a/mod/assignment/lib.php
+++ b/mod/assignment/lib.php
@@ -71,7 +71,7 @@ class assignment_base {
             error('assignment ID was incorrect');
         }
 
-        $this->assignment->cmidnumber = $this->cm->id;     // compatibility with modedit assignment obj
+        $this->assignment->cmidnumber = $this->cm->idnumber; // compatibility with modedit assignment obj
         $this->assignment->courseid   = $this->course->id; // compatibility with modedit assignment obj
 
         $this->strassignment = get_string('modulename', 'assignment');