can not get user info after doing authentication,facebookSDK, ios - objective-c

Question

I am trying to authenticate using facebook account by following
- (IBAction)logInByFaceBook:(id)sender {
NSLog(#"appDelegate.session.isOpen = %#",appDelegate.session.isOpen == 0 ? #"NO" : #"YES");
FBSessionLoginBehavior behavior = FBSessionLoginBehaviorForcingWebView;
appDelegate.session = [[FBSession alloc] init];
[appDelegate.session openWithBehavior:behavior
completionHandler:^(FBSession *session, FBSessionState status, NSError *error) {
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setObject:[NSNumber numberWithBool:YES] forKey:#"didLogIn"];
data = [[SavedData alloc] initLogInInfo:dict];
[data saveLogInInfoToDisk];
if (session.isOpen) {
FBRequest *me = [FBRequest requestForMe];
NSLog(#"me is %#",[me description]);
[me startWithCompletionHandler: ^(FBRequestConnection *connection, NSDictionary<FBGraphUser> *my, NSError *error) {
NSLog(#"my is %#",[my description]);
}
];
}
[self dismissViewControllerAnimated:YES completion:nil];
}
];
}
However, i cant not get my username back and the console is displaying
Error: HTTP status code: 400
my is (null)
By looking up this documentation, 400 means Invalid email address..... and my is null so that I can no extract the username at all.....
I dont know why this happens...I try with the same email address with facebookSDK sample code and it works properly...
Getting lost now....Can somebody point out what I am doing wrong or missing something.