protected function _buildLdapObject()
    {
        $ldap = $this->_config['ldap'];

        if (is_string($ldap)) {
            $class = App::className($ldap, 'Identifier/Ldap');
            $ldap = new $class();
        }

        if (!($ldap instanceof AdapterInterface)) {
            $message = sprintf('Option `ldap` must implement `%s`.', AdapterInterface::class);
            throw new RuntimeException($message);
        }

        $this->_ldap = $ldap;
    }