diff --git a/question/type/multianswer/question.php b/question/type/multianswer/question.php
index a52eec83b05..dd0145f96bd 100644
--- a/question/type/multianswer/question.php
+++ b/question/type/multianswer/question.php
@@ -365,6 +365,9 @@ class qtype_multianswer_question extends question_graded_automatically_with_coun
 
             $fractionsum += $subq->defaultmark * max(0, $subfraction - $lastchange * $this->penalty);
         }
+        if (empty($fractionsum)) {
+            return null;
+        }
 
         return $fractionsum / $fractionmax;
     }
diff --git a/question/type/multianswer/tests/question_test.php b/question/type/multianswer/tests/question_test.php
index 5d0beed7a8f..c386ed88fe9 100644
--- a/question/type/multianswer/tests/question_test.php
+++ b/question/type/multianswer/tests/question_test.php
@@ -367,6 +367,8 @@ class question_test extends \advanced_testcase {
 
         $this->assertEquals(1, $question->get_max_fraction());
         $this->assertEquals(0, $question->get_min_fraction());
+        $this->assertNull($question->compute_final_grade([['sub1_answer' => 'Something']], 1));
+        $this->assertNull($question->grade_response([['sub1_answer' => 'Something']])[0]);
     }
 
 }