diff --git a/mod/assignment/lib.php b/mod/assignment/lib.php
index a65441fd04017..0da882757dd6f 100644
--- a/mod/assignment/lib.php
+++ b/mod/assignment/lib.php
@@ -627,7 +627,7 @@ function submissions($mode) {
 
         if (is_null($mailinfo)) {
             if (optional_param('sesskey', null, PARAM_BOOL)) {
-                set_user_preference('assignment_mailinfo', $mailinfo);
+                set_user_preference('assignment_mailinfo', 0);
             } else {
                 $mailinfo = get_user_preferences('assignment_mailinfo', 0);
             }