--- moodle\question\backuplib.php
+++ moodle\question\backuplib_new.php
@@ -496,6 +496,10 @@
         if ($questionids){
             foreach ($questionids as $question_bk){
                 $question = get_record('question', 'id', $question_bk->old_id);
+                if (!isset($QTYPES[$question->qtype])) {
+                    notify("Question type not defined: $question->qtype");
+                    die;
+                }
                 $QTYPES[$question->qtype]->get_question_options($question);
                 $urls = array_merge_recursive($urls, $QTYPES[$question->qtype]->find_file_links($question, SITEID));
             }