diff --git a/question/engine/datalib.php b/question/engine/datalib.php
index b4b73bb749b..79506b12ef5 100644
--- a/question/engine/datalib.php
+++ b/question/engine/datalib.php
@@ -149,6 +149,7 @@ class question_engine_data_mapper {
 
     /**
      * Helper method used by insert_question_attempt_step and update_question_attempt_step
+     *
      * @param question_attempt_step $step the step to store.
      * @param int $questionattemptid the question attept id this step belongs to.
      * @param int $seq the sequence number of this stop.
@@ -158,7 +159,7 @@ class question_engine_data_mapper {
         $record = new stdClass();
         $record->questionattemptid = $questionattemptid;
         $record->sequencenumber = $seq;
-        $record->state = (string) $step->get_state();
+        $record->state = $step->get_state() ?? $step->get_state()->__toString() : null;
         $record->fraction = $step->get_fraction();
         $record->timecreated = $step->get_timecreated();
         $record->userid = $step->get_user_id();
diff --git a/question/engine/tests/datalib_test.php b/question/engine/tests/datalib_test.php
index 192aa24f892..3e46cb6bb65 100644
--- a/question/engine/tests/datalib_test.php
+++ b/question/engine/tests/datalib_test.php
@@ -40,6 +40,7 @@ require_once(__DIR__ . '/helpers.php');
  * @category  test
  * @copyright 2014 The Open University
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers \question_engine_data_mapper
  */
 class datalib_test extends \qbehaviour_walkthrough_test_base {
 
@@ -250,4 +251,32 @@ class datalib_test extends \qbehaviour_walkthrough_test_base {
         // Delete it.
         question_engine::delete_questions_usage_by_activity($quba->get_id());
     }
+
+    public function test_cannot_save_a_step_with_a_missing_state(): void {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        // Create a question.
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_question');
+        $cat = $generator->create_question_category();
+        $questiondata = $generator->create_question('shortanswer', null, ['category' => $cat->id]);
+
+        // Create a usage.
+        $quba = question_engine::make_questions_usage_by_activity('test', \context_system::instance());
+        $quba->set_preferred_behaviour('deferredfeedback');
+        $slot = $quba->add_question(question_bank::load_question($questiondata->id));
+        $quba->start_all_questions();
+
+        // Add a step with a bad state.
+        $newstep = new \question_attempt_step();
+        $newstep->set_state(null);
+        $addstepmethod = new \ReflectionMethod('question_attempt', 'add_step');
+        $addstepmethod->setAccessible(true);
+        $addstepmethod->invoke($quba->get_question_attempt($slot), $newstep);
+
+        // Verify that trying to save this throws an exception.
+        $this->expectException(\dml_write_exception::class);
+        question_engine::save_questions_usage_by_activity($quba);
+    }
 }