Symfony2和Facebook App。 CanvasHelper的AccessToken为空

I am developing a Facebook app using Symfony2. In my index route (indexAction), I ask the user to login and then, I set the access token in this way in order to retrieve some info:

$helper = $fb->getCanvasHelper();
$accessToken = $helper->getAccessToken();
$fb->setDefaultAccessToken($accessToken);

Until here, everything works fine. The problem comes when I do a redirect in this way to other page of my app from my indexAction:

return $this->redirect($route, 301);

In the new page, if I try to execute the previous code to get the access token again, I get this message:

The default access token must be of type \"string\" or Facebook\\AccessToken at /home/promocionescentros/www/vendor/facebook/php-sdk-v4/src/Facebook/Facebook.php:298)"} []

Why is happening this? Can I only get the access token one time?

Thanks

Just replace:

$accessToken = $helper->getAccessToken();

with:

$accessToken = (string)$helper->getAccessToken();