diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index 7bb0361c2cb..b7325e2afbd 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -2509,8 +2509,10 @@ class mod_assign_external extends external_api {
                         'submissionsenabled' => new external_value(PARAM_BOOL, 'Whether submissions are enabled or not.'),
                         'submissionssubmittedcount' => new external_value(PARAM_INT, 'Number of submissions in submitted status.'),
                         'submissionsneedgradingcount' => new external_value(PARAM_INT, 'Number of submissions that need grading.'),
-                        'warnofungroupedusers' => new external_value(PARAM_BOOL, 'Whether we need to warn people that there
-                                                                        are users without groups.'),
+                        'warnofungroupedusers' => new external_value(PARAM_ALPHA, 'Whether we need to warn people that there
+                                                                        are users without groups (\'warningrequired\'), warn
+                                                                        people there are users who will submit in the default
+                                                                        group (\'warningoptional\') or no warning (\'\').'),
                     ), 'Grading information.', VALUE_OPTIONAL
                 ),
                 'lastattempt' => new external_single_structure(
diff --git a/mod/assign/renderable.php b/mod/assign/renderable.php
index 27c40438dc2..86c3787a6ef 100644
--- a/mod/assign/renderable.php
+++ b/mod/assign/renderable.php
@@ -754,7 +754,7 @@ class assign_grading_summary implements renderable {
     public $isvisible = true;
 
     /** @var string no warning needed about group submissions */
-    const WARN_GROUPS_NO = 'nowarning';
+    const WARN_GROUPS_NO = false;
     /** @var string warn about group submissions, as groups are required */
     const WARN_GROUPS_REQUIRED = 'warnrequired';
     /** @var string warn about group submissions, as some will submit as 'Default group' */
diff --git a/mod/assign/tests/externallib_test.php b/mod/assign/tests/externallib_test.php
index 30ebeb5769a..dda4e5ee019 100644
--- a/mod/assign/tests/externallib_test.php
+++ b/mod/assign/tests/externallib_test.php
@@ -2029,7 +2029,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $this->assertEquals(0, $result['gradingsummary']['submissiondraftscount']);
         $this->assertEquals(1, $result['gradingsummary']['submissionssubmittedcount']);  // One student from G1 submitted.
         $this->assertEquals(1, $result['gradingsummary']['submissionsneedgradingcount']);    // One student from G1 submitted.
-        $this->assertFalse($result['gradingsummary']['warnofungroupedusers']);
+        $this->assertEmpty($result['gradingsummary']['warnofungroupedusers']);
 
         // Second group.
         $result = mod_assign_external::get_submission_status($assign->get_instance()->id, 0, $g2->id);