diff --git a/admin/webservice/forms.php b/admin/webservice/forms.php
index 289dcc4d093..745e9e186b4 100644
--- a/admin/webservice/forms.php
+++ b/admin/webservice/forms.php
@@ -163,6 +163,7 @@ class external_service_functions_form extends moodleform {
 
         $mform->addElement('searchableselector', 'fids', get_string('name'),
                 $functions, array('multiple'));
+        $mform->addRule('fids', get_string('required'), 'required', null, 'client');
 
         $mform->addElement('hidden', 'id');
         $mform->setType('id', PARAM_INT);