public function checkResponse($response) {
    if (!is_array($response)) {
      return FALSE;
    }

    $responseParams = array();
    foreach ($this->responseFields as $k => $mandatory) {
      if (isset($response[$k])) {
        $responseParams[$k] = $response[$k];
      }
      elseif (!isset($response[$k]) && $mandatory) {
        throw new \Exception('expected response field ' . $k . ' is missing');
      }
    }

    return $responseParams;
  }