diff --git a/mod/quiz/report/overview/report.php b/mod/quiz/report/overview/report.php
index d2c15952d4b..75865a6fb9d 100644
--- a/mod/quiz/report/overview/report.php
+++ b/mod/quiz/report/overview/report.php
@@ -387,7 +387,7 @@ class quiz_report extends quiz_default_report {
                             $select .= ', grade ';
                             $from        .= ' LEFT JOIN '.$CFG->prefix.'question_sessions qns ON qns.attemptid = qa.id '.
                                                 'LEFT JOIN '.$CFG->prefix.'question_states qs ON qs.id = qns.newgraded ';
-                            $where       .= ' AND ('.sql_isnull('qns.questionid').' OR qns.questionid = '.$qid.')';
+                            $where       .= ' AND (qns.questionid IS NULL OR qns.questionid = '.$qid.')';
                             $newsort[]    = 'grade '.(strpos($sortpart, 'ASC')? 'ASC' : 'DESC');
                             $questionsort = true;
                         }
@@ -668,4 +668,4 @@ document.getElementById("noscriptmenuaction").style.display = "none";
     }
 }
 
-?>
\ No newline at end of file
+?>