Facebook errors: fb:redirect: redirect forbidden

I'm working on putting a flash app onto facebook and had this error when I tried to use a page in a tab:

fb:redirect: redirect forbidden by flavor TabFBMLFlavor on the profile tab.

A bit of searching revealed that you can't require ogin: $userid = $facebook->require_login();

Unless you check to see if the userid is null:

$userid = $facebook->get_profile_user();
if(is_null($userid)){
$userid = $facebook->require_login();
}