private function _validateId($identifier = null, $identifierType = 'token')
    {
        if (empty($identifier)) {
           throw new InvalidArgumentException(
                   'expected paypal account id to be set'
                   );
        }
        if (!preg_match('/^[0-9A-Za-z_-]+$/', $identifier)) {
            throw new InvalidArgumentException(
                    $identifier . ' is an invalid paypal account ' . $identifierType . '.'
                    );
        }
    }