private function errorMessage($rule, $param = null)
    {
        if ($rule == 'remote' && isset($this->errors[$param])) {
            return $this->errorMessage($param);
        }
        if (is_null($param)) {
            $param = '';
        }
        $params = array_pad((array) $param, 2, '');

        return (isset($this->errors[$rule])) ? str_replace(array('{0}', '{1}'), $params, $this->errors[$rule]) : null;
    }