How do I add a hover state to a smart shape button on the master slide? CP7

I want to use a button with a hover state on my master slide in CP7. I created a smart shape and checked the box 'use as button.' I then created 3 image files with the "x_up, x_down, x_over' extensions. When I select my smart shape and choose a custom pattern from the fill menu and select my 'x_up' image file, the smart shape does not recognize the hover state. Is there another way to achieve this?

Thanks!