public function callError($id, $errorUri, $desc = '', $details = null) {
        if ($errorUri instanceof Topic) {
            $errorUri = (string)$errorUri;
        }

        $data = array(WAMP::MSG_CALL_ERROR, $id, $errorUri, $desc);

        if (null !== $details) {
            $data[] = $details;
        }

        return $this->send(json_encode($data));
    }