diff --git a/lib/classes/grades_external.php b/lib/classes/grades_external.php
index 0c58ce7a3ce..5781accbe3c 100644
--- a/lib/classes/grades_external.php
+++ b/lib/classes/grades_external.php
@@ -405,7 +405,7 @@ class core_grades_external extends external_api {
                             'str_feedback' => new external_value(
                                 PARAM_TEXT, 'A string representation of the feedback from the grader', VALUE_OPTIONAL),
                         )
-                ), 'Any student grades to alter', VALUE_OPTIONAL),
+                ), 'Any student grades to alter', VALUE_DEFAULT, array()),
                 'itemdetails' => new external_single_structure(
                     array(
                         'itemname' => new external_value(
@@ -428,7 +428,7 @@ class core_grades_external extends external_api {
                             PARAM_BOOL, 'True if the grade item should be deleted', VALUE_OPTIONAL),
                         'hidden' => new external_value(
                             PARAM_BOOL, 'True if the grade item is hidden', VALUE_OPTIONAL),
-                    ), 'Any grade item settings to alter', VALUE_OPTIONAL
+                    ), 'Any grade item settings to alter', VALUE_DEFAULT, array()
                 )
             )
         );
diff --git a/mod/forum/externallib.php b/mod/forum/externallib.php
index 98b144cd4f9..4ff0332482e 100644
--- a/mod/forum/externallib.php
+++ b/mod/forum/externallib.php
@@ -161,8 +161,8 @@ class mod_forum_external extends external_api {
             array(
                 'forumids' => new external_multiple_structure(new external_value(PARAM_INT, 'forum ID',
                         '', VALUE_REQUIRED, '', NULL_NOT_ALLOWED), 'Array of Forum IDs', VALUE_REQUIRED),
-                'limitfrom' => new external_value(PARAM_INT, 'limit from', VALUE_OPTIONAL, 0),
-                'limitnum' => new external_value(PARAM_INT, 'limit number', VALUE_OPTIONAL, 0)
+                'limitfrom' => new external_value(PARAM_INT, 'limit from', VALUE_DEFAULT, 0),
+                'limitnum' => new external_value(PARAM_INT, 'limit number', VALUE_DEFAULT, 0)
             )
         );
     }
diff --git a/webservice/lib.php b/webservice/lib.php
index 9a867d2fcdb..879c716bd1c 100644
--- a/webservice/lib.php
+++ b/webservice/lib.php
@@ -1313,9 +1313,9 @@ class '.$classname.' {
                         }
                     }
                 } else if ($keydesc->required == VALUE_OPTIONAL) {
-                    //it does make sens to declare a parameter VALUE_OPTIONAL
-                    //VALUE_OPTIONAL is used only for array/object key
-                    throw new moodle_exception('parametercannotbevalueoptional');
+                    // It does not make sense to declare a parameter VALUE_OPTIONAL.
+                    // VALUE_OPTIONAL is used only for array/object key.
+                    throw new moodle_exception('erroroptionalparamarray', 'webservice', '', $name);
                 }
             } else { //for the moment we do not support default for other structure types
                  if ($keydesc->required == VALUE_DEFAULT) {