diff --git a/question/engine/questionattempt.php b/question/engine/questionattempt.php
index 5473535b7c6..705594dc3f0 100644
--- a/question/engine/questionattempt.php
+++ b/question/engine/questionattempt.php
@@ -1184,7 +1184,7 @@ class question_attempt {
     protected function check_qt_var_name_restrictions($expected) {
         global $CFG;
 
-        if ($CFG->debugdeveloper) {
+        if ($CFG->debugdeveloper && $expected !== self::USE_RAW_DATA) {
             foreach ($expected as $key => $value) {
                 if (strpos($key, 'bf_') !== false) {
                     debugging('The bf_ prefix is reserved and cannot be used by question types', DEBUG_DEVELOPER);