diff --git a/mod/quiz/report/overview/report.php b/mod/quiz/report/overview/report.php
index a8e59d132e6..22f404ff35f 100644
--- a/mod/quiz/report/overview/report.php
+++ b/mod/quiz/report/overview/report.php
@@ -622,7 +622,7 @@ class quiz_overview_report extends quiz_default_report {
         global $DB, $COURSE;
         foreach($attemptids as $attemptid) {
             $attempt = $DB->get_record('quiz_attempts', array('id' => $attemptid));
-            if ($groupstudents && !in_array($attempt->userid, $groupstudents)) {
+            if ($groupstudents && !array_key_exists($attempt->userid, $groupstudents)) {
                 continue;
             }
             add_to_log($COURSE->id, 'quiz', 'delete attempt', 'report.php?id=' . $cm->id,
diff --git a/mod/quiz/report/responses/report.php b/mod/quiz/report/responses/report.php
index a485db3f01f..126ff50bb2f 100644
--- a/mod/quiz/report/responses/report.php
+++ b/mod/quiz/report/responses/report.php
@@ -115,7 +115,7 @@ class quiz_responses_report extends quiz_default_report {
             require_capability('mod/quiz:deleteattempts', $context);
             foreach ($attemptids as $attemptid) {
                 $attempt = $DB->get_record('quiz_attempts', array('id' => $attemptid));
-                if ($groupstudents && !in_array($attempt->userid, $groupstudents)) {
+                if ($groupstudents && !array_key_exists($attempt->userid, $groupstudents)) {
                     continue;
                 }
                 add_to_log($course->id, 'quiz', 'delete attempt', 'report.php?id=' . $cm->id,