来自页面选项卡的Facebook签名请求不包含用户或页面信息

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!