diff --git a/mod/forum/discuss.php b/mod/forum/discuss.php
index fa2d83fa557c1..feb0d2af4a139 100644
--- a/mod/forum/discuss.php
+++ b/mod/forum/discuss.php
@@ -77,10 +77,14 @@
     unset($SESSION->fromdiscussion);
 
     if ($mode) {
-        set_user_preference("forum_displaymode", $mode);
+        if (isguest()) {
+            $USER->preference['forum_displaymode'] = $mode;  // don't save it in database
+        } else {
+            set_user_preference('forum_displaymode', $mode);
+        }
     }
 
-    $displaymode = get_user_preferences("forum_displaymode", $CFG->forum_displaymode);
+    $displaymode = get_user_preferences('forum_displaymode', $CFG->forum_displaymode);
 
     if ($parent) {
         if (abs($displaymode) == 1) {  // If flat AND parent, then force nested display this time