iOS: UIActivityIndicator in navigation bar not stopping

I wrote small test app, that doesn't work. This is my storyboard:
_{(source: cs618122.vk.me)}

This is my code:

- (void)viewDidLoad
{
    [super viewDidLoad];
    UIActivityIndicatorView* ai = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    UIBarButtonItem* rightButton = [[UIBarButtonItem alloc] initWithCustomView:ai];
    self.navigationItem.rightBarButtonItem = rightButton;
    [ai startAnimating];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [NSThread sleepForTimeInterval: 5];
        NSLog(@"STOP!");
        [ai stopAnimating];
    });
}

This is work result:
_{(source: cs618122.vk.me)}

What's wrong?

Here is kind of the same problem solved:

[spinner startAnimating];
dispatch_async(kBgQueue, ^{
    // ... do other stuff in the background
    dispatch_async(dispatch_get_main_queue(), ^{
        [spinner stopAnimating];
   });
}); 

UIActivityIndicatorView won't stop