diff --git a/question/type/calculated/db/upgradelib.php b/question/type/calculated/db/upgradelib.php
index d4c8418..dd6d2a6 100644
--- a/question/type/calculated/db/upgradelib.php
+++ b/question/type/calculated/db/upgradelib.php
@@ -98,7 +98,7 @@ class qtype_calculated_qe2_attempt_updater extends question_qtype_attempt_update
         }
 
         if (!$realanswer) {
-            return '';
+            return array('','');
         }
 
         if (strpos($realanswer, '|||||') === false) {