public function execute($command, $params = null)
    {
        $url = $command;

        if (substr($url, -1) !== '/') {
            $url .= '/';
        }

        if ($params) {
            $parameters = array();
            foreach ($params as $k => $v) {
                if (strlen($v)) {
                    $parameters[] = urlencode($k) . '=' . urlencode($v);
                } else {
                    $parameters[] = urlencode($k);
                }
            }
            $url .= '?' . implode('&', $parameters);
        }

        try {
            $response = $this->client->get($url);
        } catch (\Exception $e) {
            return null;
        }
        $this->checkError($response, $url);

        return $response->getBody();
    }