protected function normalizeClientConfig(array $config)
    {
        if (! isset($config['client'])) {
            $config['client'] = ['url' => $config['url']];
        } elseif (is_string($config['client'])) {
            $client = Arr::get($this->config, 'clients.'.$config['client']);

            if ($client == null) {
                throw new InvalidArgumentException('Client '.$config['client'].' doesn\'t configured.');
            }

            $config['client'] = $client;
        }

        return $config;
    }