Facebook SDK: Accessing "permitted" info

[starwarsdevwookie59]

My application needs to access the birthdays of the users' friends. To do so, my app access friends_birthday, as listed in the API Documentation, and try to access it using...

params = [[NSMutableDictionary alloc] initWithObjects:
[[NSArray alloc] initWithObjects:self.facebook.accessToken, nil] forKeys:
[[NSArray alloc] initWithObjects:@"access_token", nil]];

//then I request the info

[facebook requestWithGraphPath:userID andParams:params andDelegate:self];

//this returns (null)
[result objectForKey:@"birthday"]

Anybody know what I need to do? I can access public information fine, it's just when I try to access information that I've requested permission to, I can't get anything.