diff --git a/mod/forum/classes/local/renderers/discussion.php b/mod/forum/classes/local/renderers/discussion.php
index 8b7e6249186..084122a7af7 100644
--- a/mod/forum/classes/local/renderers/discussion.php
+++ b/mod/forum/classes/local/renderers/discussion.php
@@ -284,7 +284,7 @@ class discussion {
      *
      * @return string
      */
-    private function get_move_discussion_html() : string {
+    private function get_move_discussion_html() : ?string {
         global $DB;
 
         $forum = $this->forum;