diff --git a/question/type/multianswer/questiontype.php b/question/type/multianswer/questiontype.php
index 52ca37d5f13..e8594d1547d 100644
--- a/question/type/multianswer/questiontype.php
+++ b/question/type/multianswer/questiontype.php
@@ -314,11 +314,18 @@ class qtype_multianswer extends question_type {
         $fractionsum = 0;
         $fractionmax = 0;
         foreach ($questiondata->options->questions as $key => $subqdata) {
+            if ($subqdata->qtype == 'subquestion_replacement') {
+                continue;
+            }
             $fractionmax += $subqdata->defaultmark;
             $fractionsum += question_bank::get_qtype(
                     $subqdata->qtype)->get_random_guess_score($subqdata);
         }
-        return $fractionsum / $fractionmax;
+        if ($fractionmax > question_utils::MARK_TOLERANCE) {
+            return $fractionsum / $fractionmax;
+        } else {
+            return null;
+        }
     }
 
     public function move_files($questionid, $oldcontextid, $newcontextid) {
diff --git a/question/type/multianswer/tests/question_type_test.php b/question/type/multianswer/tests/question_type_test.php
index d36a13e85a4..fe36a8b0a3a 100644
--- a/question/type/multianswer/tests/question_type_test.php
+++ b/question/type/multianswer/tests/question_type_test.php
@@ -34,9 +34,10 @@ require_once($CFG->dirroot . '/question/type/multianswer/edit_multianswer_form.p
 /**
  * Unit tests for the multianswer question definition class.
  *
- * @package    qtype_multianswer
- * @copyright  2011 The Open University
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @package   qtype_multianswer
+ * @copyright 2011 The Open University
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers    \qtype_multianswer
  */
 class question_type_test extends \advanced_testcase {
     /** @var qtype_multianswer instance of the question type class to test. */
@@ -112,15 +113,46 @@ class question_type_test extends \advanced_testcase {
     }
 
     public function test_get_random_guess_score() {
-        $q = \test_question_maker::get_question_data('multianswer', 'twosubq');
+        $q = test_question_maker::get_question_data('multianswer', 'twosubq');
         $this->assertEqualsWithDelta(0.1666667, $this->qtype->get_random_guess_score($q), 0.0000001);
     }
 
+    public function test_get_random_guess_score_with_missing_subquestion() {
+        global $DB;
+        $this->resetAfterTest();
+
+        // Create a question referring to a subquesion that has got lost (which happens some time).
+        /** @var \core_question_generator $generator */
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_question');
+        $category = $generator->create_question_category();
+        $question = $generator->create_question('multianswer', 'twosubq', ['category' => $category->id]);
+        // Add a non-existent subquestion id to the list.
+        $sequence = $DB->get_field('question_multianswer', 'sequence', ['question' => $question->id], MUST_EXIST);
+        $DB->set_field('question_multianswer', 'sequence', $sequence . ',-1', ['question' => $question->id]);
+
+        // Verify that computing the random guess score does not give an error.
+        $questiondata = question_bank::load_question_data($question->id);
+        $this->assertEqualsWithDelta(0.1666667, $this->qtype->get_random_guess_score($questiondata), 0.0000001);
+    }
+
+    public function test_get_random_guess_score_with_all_missing_subquestions() {
+        $this->resetAfterTest();
+
+        // Create a question where all subquestions are missing.
+        $questiondata = test_question_maker::get_question_data('multianswer', 'twosubq');
+        foreach ($questiondata->options->questions as $subq) {
+            $subq->qtype = 'subquestion_replacement';
+        }
+
+        // Verify that computing the random guess score does not give an error.
+        $this->assertNull($this->qtype->get_random_guess_score($questiondata));
+    }
+
     public function test_load_question() {
         $this->resetAfterTest();
 
         $syscontext = \context_system::instance();
-        /** @var core_question_generator $generator */
+        /** @var \core_question_generator $generator */
         $generator = $this->getDataGenerator()->get_plugin_generator('core_question');
         $category = $generator->create_question_category(['contextid' => $syscontext->id]);