带有NSNotification的removeObserver ...我做错了什么？

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doSomething:) name:@"SomeNotification" object:nil];
}

- (void)viewDidUnload {
    [super viewDidUnload];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:@"SomeNotification" object:nil];
}