protected function _getBearerAuthorization()
  {
    if (self::$_consumer_key === null
      && self::$_bearer_token === null
    ) {
      throw new CodebirdCredentialsException('To make an app-only auth API request, consumer key or bearer token must be set.');
    }
    // automatically fetch bearer token, if necessary
    if (self::$_bearer_token === null) {
      $this->oauth2_token();
    }
    return 'Bearer ' . self::$_bearer_token;
  }