diff --git a/mod/forum/post.php b/mod/forum/post.php
index bdbe12aeea9..fc0f0bdf13e 100644
--- a/mod/forum/post.php
+++ b/mod/forum/post.php
@@ -255,7 +255,8 @@
 
         if ($cm = get_coursemodule_from_instance("forum", $forum->id, $course->id)) {
             if (groupmode($course, $cm) and !isteacheredit($course->id)) {   // Make sure user can post here
-                if (mygroupid($course->id) != $discussion->groupid) {
+                $mygroupid = mygroupid($course->id);
+                if (!((empty($mygroupid) and $discussion->groupid == -1) || ($mygroupid == $discussion->groupid))) {
                     error("Sorry, but you can not post in this discussion.");
                 }
             }