Loading GameCenter Log-in in ViewDidLoad

I researched Apple's documentation for GameCenter, but when I tried to implement their sample code, it did not work. Am I doing something incorrectly?

 - (void)viewDidLoad {
[super viewDidLoad];

 GKGameCenterViewController *gameCenterController = [[GKGameCenterViewController alloc] init];
if (gameCenterController != nil)
{
   gameCenterController.gameCenterDelegate = self;
   [self presentViewController: gameCenterController animated: YES completion:nil];
}
}

I am trying to load the sign in for gamecenter right when the user opens the app. Thank you