Modal View Controller with ARC never gets released
===

When dismissing a modal view controller, the controller is supposed to be released. But it's not, so I set up a test with the simplest code possible:

1. a new project with a single view, with ARC enabled.
2. add a button to the single view, when clicked, create a UIViewController with a white blank view, and present it modally.
3. Set a timer to close the modal view controller automatically after 5 seconds.

Here's all the code I added besides xcode template:

    - (IBAction)showModalView:(id)sender
    {
        UIViewController *mvc = [[UIViewController alloc] init];
        [[wvc view] setFrame:CGRectMake(0, 0, 320, 460)];
        [[wvc view] setBackgroundColor:[UIColor whiteColor]];
        
        [self presentViewController:mvc animated:NO completion:nil];
    
        NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:4 target:self selector:@selector(dismissModalView) userInfo:nil repeats:NO];
        NSRunLoop *mainLoop = [NSRunLoop currentRunLoop];
        [mainLoop addTimer:timer forMode:NSDefaultRunLoopMode];
    }

    - (void)dismissModalView
    {
        [self dismissViewControllerAnimated:NO completion:nil];
    }

When I run this simplest project with Zombie instrument, it shows the modal view controller has a retain count of 0, yet it's not released when dismissed. I've tried every trick in my possession to no avail. 

Please Help.....