private function assertAuthSite($authSite)
    {
        if (!array_key_exists($authSite, $this->authSites)) {
            $validValues = array_keys($this->authSites);
            $validValues = implode(', ', $validValues);
            $template = 'Valid values for authSite are only: %s';
            $message = sprintf($template, $validValues);
            throw new InvalidArgumentException($message);
        }
    }