/**
  * Determine if we have an authentication failure, and, if so, return a 401 response
  *
  * @param AccessControlEvent $accessControlEvent
  * @return \Parrot\API\Problem\Response\ProblemResponse|\Zend\Stdlib\ResponseInterface
  */
 public function __invoke(AccessControlEvent $accessControlEvent)
 {
     $result = $accessControlEvent->getAuthenticationResult();
     if ($result->isValid()) {
         return;
     }
     $mvcEvent = $accessControlEvent->getMvcEvent();
     $response = $mvcEvent->getResponse();
     if (!$response instanceof HttpResponse) {
         return $response;
     }
     return new ProblemResponse(new Problem(401, 'Unauthorized'));
 }