protected function _create($className, $alias, $config)
    {
        $authenticator = new $className($this->_identifiers, $config);
        if (!($authenticator instanceof AuthenticatorInterface)) {
            throw new RuntimeException(sprintf(
                'Authenticator class `%s` must implement \Auth\Authentication\AuthenticatorInterface',
                $className
            ));
        }

        return $authenticator;
    }