向UIImageView添加一个子视图，它是IB中UIViewController的视图出口

I have a UIViewController object in a .xib file. I add a UIImageView to it, and then try to add a button as a subview of the UIImageView. It instead replaces the UIImageView. It works fine if both are subviews of a UIView. What gives? I thought this would work since UIImageView is a subclass of UIView?I have a UIViewController object in a .xib file