diff --git a/mod/assignment/lib.php b/mod/assignment/lib.php
index 6d8c0cf39e7..f79fba49b24 100644
--- a/mod/assignment/lib.php
+++ b/mod/assignment/lib.php
@@ -255,8 +255,10 @@ function assignment_grades($assignmentid) {
         $scaleid = - ($assignment->grade);
         if ($scale = get_record("scale", "id", $scaleid)) {
             $scalegrades = make_menu_from_list($scale->scale);
-            foreach ($grades as $key => $grade) {
-                $grades[$key] = $scalegrades[$grade];
+            if ($grades) {
+                foreach ($grades as $key => $grade) {
+                    $grades[$key] = $scalegrades[$grade];
+                }
             }
         }
         $return->grades = $grades;