diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index 92d0e83001a..adb0a7996bd 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -2775,7 +2775,9 @@ class mod_assign_external extends external_api {
         // Skip the expensive lookup of user detail if we're blind marking or the caller
         // hasn't asked for user details to be embedded.
         if (!$assign->is_blind_marking() && $embeduser) {
-            $return['user'] = user_get_user_details($participant, $course);
+            if ($userdetails = user_get_user_details($participant, $course)) {
+                $return['user'] = $userdetails;
+            }
         }
 
         return $return;
diff --git a/mod/assign/tests/externallib_test.php b/mod/assign/tests/externallib_test.php
index 38906920540..e5a0ccf50ac 100644
--- a/mod/assign/tests/externallib_test.php
+++ b/mod/assign/tests/externallib_test.php
@@ -2203,6 +2203,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->setUser($teacher);
 
         $result = mod_assign_external::get_participant($assign->id, $student->id, false);
+        $result = external_api::clean_returnvalue(mod_assign_external::get_participant_returns(), $result);
     }
 
     /**
@@ -2224,6 +2225,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->setUser($teacher);
 
         $result = mod_assign_external::get_participant($assign->id, $student->id, true);
+        $result = external_api::clean_returnvalue(mod_assign_external::get_participant_returns(), $result);
         $this->assertEquals($student->id, $result['id']);
         $this->assertFalse(fullname($student) == $result['fullname']);
         $this->assertFalse($result['submitted']);
@@ -2231,7 +2233,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->assertFalse($result['grantedextension']);
         $this->assertTrue($result['blindmarking']);
         // Make sure we don't get any additional info.
-        $this->assertTrue(empty($result['user']));
+        $this->assertArrayNotHasKey('user', $result);
     }
 
     /**
@@ -2277,6 +2279,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->setUser($teacher);
 
         $result = mod_assign_external::get_participant($assignmodule->id, $student->id, false);
+        $result = external_api::clean_returnvalue(mod_assign_external::get_participant_returns(), $result);
         $this->assertEquals($student->id, $result['id']);
         $this->assertEquals(fullname($student), $result['fullname']);
         $this->assertTrue($result['submitted']);
@@ -2284,7 +2287,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->assertFalse($result['grantedextension']);
         $this->assertFalse($result['blindmarking']);
         // Make sure we don't get any additional info.
-        $this->assertTrue(empty($result['user']));
+        $this->assertArrayNotHasKey('user', $result);
     }
 
     /**
@@ -2305,6 +2308,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->setUser($teacher);
 
         $result = mod_assign_external::get_participant($assign->id, $student->id, true);
+        $result = external_api::clean_returnvalue(mod_assign_external::get_participant_returns(), $result);
         // Check some of the extended properties we get when requesting the user.
         $this->assertEquals($student->id, $result['id']);
         // We should get user infomation back.
@@ -2356,6 +2360,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->setUser($teacher);
 
         $result = mod_assign_external::get_participant($assignmodule->id, $student->id, false);
+        $result = external_api::clean_returnvalue(mod_assign_external::get_participant_returns(), $result);
         // Check some of the extended properties we get when not requesting a summary.
         $this->assertEquals($student->id, $result['id']);
         $this->assertEquals($group->id, $result['groupid']);
@@ -2390,6 +2395,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
 
         $this->setUser($teacher);
         $participants = mod_assign_external::list_participants($assignment->id, 0, '', 0, 0, false, true);
+        $participants = external_api::clean_returnvalue(mod_assign_external::list_participants_returns(), $participants);
         $this->assertCount(1, $participants);
 
         // Asser that we have a valid response data.
@@ -2407,6 +2413,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->assertArrayHasKey('enrolledcourses', $participant);
 
         $participants = mod_assign_external::list_participants($assignment->id, 0, '', 0, 0, false, false);
+        $participants = external_api::clean_returnvalue(mod_assign_external::list_participants_returns(), $participants);
         // Check that the list of courses the participant is enrolled is not returned.
         $participant = $participants[0];
         $this->assertArrayNotHasKey('enrolledcourses', $participant);