diff --git a/question/type/multichoice/db/upgradelib.php b/question/type/multichoice/db/upgradelib.php
index 7a197b1bbf766..0b9b2e7bcf0a0 100644
--- a/question/type/multichoice/db/upgradelib.php
+++ b/question/type/multichoice/db/upgradelib.php
@@ -41,7 +41,7 @@ class qtype_multichoice_qe2_attempt_updater extends question_qtype_attempt_updat
 
     public function is_blank_answer($state) {
         // blank multichoice answers are not empty strings, they rather end in a colon
-        return empty($state->answer) || substr($a, -1) == ':';
+        return empty($state->answer) || substr($state->answer, -1) == ':';
     }
 
     public function right_answer() {