IOS: Weird color in UIView transition : UIViewAnimationOptionTransitionCurlDown

I'm really at a loss here, I am trying to use the UIViewAnimationOptionTransitionCurlDown on a view that has some transparency, and here is the result (visually), the code is below. What I want is for the transition to happen without the weird shadow. Any insight as to why that shadow i开发者_如何学Gos displaying would also be helpful. It only happens during the animation.

    [UIView transitionWithView:sender 
                  duration:15.0f 
                   options:UIViewAnimationOptionTransitionCurlDown
                animations:^{
                    [self modifyContentOfPageWith:sender];
                } 
                completion:nil]; 

I tried the same piece of code. Downloaded a yellow sticky image from the web and realized that the actual size of the image is bigger than the active image. Thus a shade of the whole image was during the animation. So I cut the (invisible) borders of the image that it would fit the yellow part and , magically, worked just perfect.