private function registerProxySources($apiPrefix = '')
    {
        foreach (array_keys($this->proxySourceConfiguration) as $source) {
            foreach ($this->proxySourceConfiguration[$source] as $config) {
                if ($apiPrefix == $config['prefix']) {
                    $this->apiLoader->setOption($config);
                    return;
                }
            }
        }

        $e = new NotFoundException('No such thirdparty API.');
        $e->setResponse(Response::create());
        throw $e;
    }