diff --git a/classes/modules/turnitin_quiz.class.php b/classes/modules/turnitin_quiz.class.php
index 059f03ef..75519115 100644
--- a/classes/modules/turnitin_quiz.class.php
+++ b/classes/modules/turnitin_quiz.class.php
@@ -82,7 +82,8 @@ public function update_mark($attemptid, $identifier, $userid, $grade, $quizgrade
         foreach ($attempt->get_slots() as $slot) {
             $answer = $attempt->get_question_attempt($slot)->get_response_summary();
             // Check if this is the slot the mark is for by matching content.
-            if (sha1($answer.$slot) == $identifier) {
+            $answerslot = $answer ? $answer.$slot : $answer;
+            if (sha1($answerslot) == $identifier) {
                 // Translate the TFS grade to a mark for the question.
                 $questionmaxmark = $attempt->get_question_attempt($slot)->get_max_mark();