diff --git a/lib/externallib.php b/lib/externallib.php
index 0c6e44dbc04..abde8a28e47 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -598,15 +598,15 @@ class external_warnings extends external_multiple_structure {
      *
      * @since Moodle 2.3
      */
-    public function __construct() {
+    public function __construct($itemdesc = 'item', $itemiddesc = 'item id',
+        $warningcodedesc = 'the warning code can be used by the client app to implement specific behaviour') {
 
         parent::__construct(
             new external_single_structure(
                 array(
-                    'item' => new external_value(PARAM_TEXT, 'item', VALUE_OPTIONAL),
-                    'itemid' => new external_value(PARAM_INT, 'item id', VALUE_OPTIONAL),
-                    'warningcode' => new external_value(PARAM_ALPHANUM,
-                            'the warning code can be used by the client app to implement specific behaviour'),
+                    'item' => new external_value(PARAM_TEXT, $itemdesc, VALUE_OPTIONAL),
+                    'itemid' => new external_value(PARAM_INT, $itemiddesc, VALUE_OPTIONAL),
+                    'warningcode' => new external_value(PARAM_ALPHANUM, $warningcodedesc),
                     'message' => new external_value(PARAM_TEXT,
                             'untranslated english message to explain the warning')
                 ), 'warning'),
diff --git a/mod/assign/externallib.php b/mod/assign/externallib.php
index a7a2e80014e..07de54fbaf6 100644
--- a/mod/assign/externallib.php
+++ b/mod/assign/externallib.php
@@ -184,7 +184,9 @@ class mod_assign_external extends external_api {
         return new external_single_structure(
             array(
                 'assignments' => new external_multiple_structure(self::assign_grades(), 'list of assignment grade information'),
-                'warnings'      => new external_warnings()
+                'warnings'      => new external_warnings('item is always \'assignment\'',
+                    'when errorcode is 3 then itemid is an assignment id. When errorcode is 1, itemid is a course module instance id',
+                    'errorcode can be 3 (no grades found) or 1 (no permission to get grades)')
             )
         );
     }
@@ -425,7 +427,9 @@ class mod_assign_external extends external_api {
         return new external_single_structure(
             array(
                 'courses' => new external_multiple_structure(self::get_assignments_course_structure(), 'list of courses'),
-                'warnings'  => new external_warnings()
+                'warnings'  => new external_warnings('item can be \'course\' (errorcode 1 or 2) or \'module\' (errorcode 1)',
+                    'When item is a course then itemid is a course id. When the item is a module then itemid is a module id',
+                    'errorcode can be 1 (no access rights) or 2 (not enrolled or no permissions)')
             )
         );
     }