为什么UIActionSheet取消会使我的应用程序崩溃?

- (void) doItWithConfirm {

    UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:@"do you really wanna do it?" 
                            delegate:self cancelButtonTitle:@"I don't" destructiveButtonTitle: nil
                            otherButtonTitles:@"Yes, do it", nil];

    [actionSheet showInView:self.view];
    [actionSheet release];
}

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex {

    NSLog(@"buttonIndex: %@", buttonIndex);
}