public function getField($id, $field)
    {
        if (!in_array($field, static::$fields)) {
            throw new InvalidArgumentException(sprintf('There is no field named %s.', $field));
        }

        $response = $this->get($this->path.'/'.rawurlencode($id).'/'.rawurlencode($field));

        return isset($response['_value']) ? $response['_value'] : $response;
    }