diff --git a/question/engine/datalib.php b/question/engine/datalib.php
index 26af89ade9f..7d5f1804202 100644
--- a/question/engine/datalib.php
+++ b/question/engine/datalib.php
@@ -100,10 +100,7 @@ class question_engine_data_mapper {
             $stepdata[] = $this->insert_question_attempt($qa, $quba->get_owning_context());
         }
 
-        $stepdata = call_user_func_array('array_merge', $stepdata);
-        if ($stepdata) {
-            $this->insert_all_step_data($stepdata);
-        }
+        $this->insert_all_step_data($this->combine_step_data($stepdata));
 
         $quba->set_observer(new question_engine_unit_of_work($quba));
     }
@@ -150,7 +147,7 @@ class question_engine_data_mapper {
             $stepdata[] = $this->insert_question_attempt_step($step, $record->id, $seq, $context);
         }
 
-        return call_user_func_array('array_merge', $stepdata);
+        return $this->combine_step_data($stepdata);
     }
 
     /**
@@ -171,6 +168,22 @@ class question_engine_data_mapper {
         return $record;
     }
 
+    /**
+     * Take an array of arrays, and flatten it, even if the outer array is empty.
+     *
+     * Only public so it can be called from the unit of work. Not part of the
+     * public API of this class.
+     *
+     * @param array $stepdata array of zero or more arrays.
+     * @return array made by concatenating all the separate arrays.
+     */
+    public function combine_step_data(array $stepdata): array {
+        if (empty($stepdata)) {
+            return [];
+        }
+        return call_user_func_array('array_merge', $stepdata);
+    }
+
     /**
      * Helper method used by insert_question_attempt_step and update_question_attempt_step
      * @param question_attempt_step $step the step to store.
@@ -1581,9 +1594,7 @@ class question_engine_unit_of_work implements question_usage_observer {
             $dm->update_questions_usage_by_activity($this->quba);
         }
 
-        if ($stepdata) {
-            $dm->insert_all_step_data(call_user_func_array('array_merge', $stepdata));
-        }
+        $dm->insert_all_step_data($dm->combine_step_data($stepdata));
 
         $this->stepsdeleted = array();
         $this->stepsmodified = array();
diff --git a/question/engine/tests/datalib_test.php b/question/engine/tests/datalib_test.php
index 42b938e03d8..d00d260759a 100644
--- a/question/engine/tests/datalib_test.php
+++ b/question/engine/tests/datalib_test.php
@@ -230,4 +230,25 @@ class question_engine_data_mapper_testcase extends qbehaviour_walkthrough_test_b
         $this->assertEquals(0, $DB->count_records('question_attempts') - $initialqarows);
         $this->assertEquals(2, $DB->count_records('question_attempt_steps') - $initialqasrows);
     }
+
+    /**
+     * Test that database operations on an empty usage work without errors.
+     */
+    public function test_save_and_load_an_empty_usage() {
+        $this->resetAfterTest();
+
+        // Create a new usage.
+        $quba = question_engine::make_questions_usage_by_activity('test', context_system::instance());
+        $quba->set_preferred_behaviour('deferredfeedback');
+
+        // Save it.
+        question_engine::save_questions_usage_by_activity($quba);
+
+        // Reload it.
+        $reloadedquba = question_engine::load_questions_usage_by_activity($quba->get_id());
+        $this->assertCount(0, $quba->get_slots());
+
+        // Delete it.
+        question_engine::delete_questions_usage_by_activity($quba->get_id());
+    }
 }