diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index 92ce5961709..efe7ecc0a29 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -2602,11 +2602,12 @@ class mod_assign_external extends external_api {
      * @param string $filter search string to filter the results.
      * @param int $skip Number of records to skip
      * @param int $limit Maximum number of records to return
+     * @param bool $onlyids Only return user ids.
      * @return array of warnings and status result
      * @since Moodle 3.1
      * @throws moodle_exception
      */
-    public static function list_participants($assignid, $groupid, $filter, $skip, $limit) {
+    public static function list_participants($assignid, $groupid, $filter, $skip, $limit, $onlyids) {
         global $DB, $CFG;
         require_once($CFG->dirroot . "/mod/assign/locallib.php");
         require_once($CFG->dirroot . "/user/lib.php");
@@ -2617,7 +2618,8 @@ class mod_assign_external extends external_api {
                                                 'groupid' => $groupid,
                                                 'filter' => $filter,
                                                 'skip' => $skip,
-                                                'limit' => $limit
+                                                'limit' => $limit,
+                                                'onlyids' => $onlyids
                                             ));
         $warnings = array();
 
diff --git a/mod/assign/tests/externallib_test.php b/mod/assign/tests/externallib_test.php
index ca1e96e4ffc..bac829360e3 100644
--- a/mod/assign/tests/externallib_test.php
+++ b/mod/assign/tests/externallib_test.php
@@ -2294,7 +2294,7 @@ class mod_assign_external_testcase extends externallib_advanced_testcase {
         $DB->update_record('user', $student);
 
         $this->setUser($teacher);
-        $participants = mod_assign_external::list_participants($assignment->id, 0, '', 0, 0);
+        $participants = mod_assign_external::list_participants($assignment->id, 0, '', 0, 0, false);
         $this->assertCount(1, $participants);
 
         // Asser that we have a valid response data.