Facebook signed request from page tab contains no user or page info

Facebook signed request from page tab contains no user or page info

I'm having some trouble with a page tab that I created. It's supposed to
show the content in two different languages (Dutch and French) depending
on the user's locale. So normally I would read out the signed request
given by Facebook and check the locale there...
However, the user info array is missing from the signed request. The
result looks like this:
array(4) { ["algorithm"]=> string(11) "HMAC-SHA256" ["code"]=> string(323)
"xxxx" ["issued_at"]=> int(1378751660) ["user_id"]=> string(9) "651076788"
}
As you can see, no user or page info available.
I'm not really sure what's going on here. I'm storing the signed request
in a session the first time I encounter it and it's not null (screenshot
1: http://i.stack.imgur.com/IGbbf.png)
Also, I made sure that the page tab url's end with a trailing slash
(screenshot 2: http://i.stack.imgur.com/fAyNr.png)
So I'm really not sure where this is going wrong...
Any help would be greatly appriciated!