Elements rendered by PIE can not have their opacity changed, so fading won't work. You should be able to wrap it in another element and do the fade on that outer element.

Why can elements with PIE not have opacity?

There are technical limitations that have prevented it:
- IE's lack of true opacity support means you have to use their proprietary alpha() filter, which has very poor rendering quality when used on things like antialiased text.
- PIE's rendering involves two layered elements, which means that they cannot be faded as a single unit; the best we could do would be to fade each one individually, which is unfortunately not the same result.