Buttons under a mask layer

I'm building a website with a Flash menu. The buttons are animated, and I used a mask layer over them to create part of the effect.
The buttons all have actions like this:

on(rollOver) {
      gotoAndPlay(1);
}
on(rollOut) {
      gotoAndPlay(_totalframes-_currentframe);
}
on(release) {
      getURL("mypage.asp", _parent);
}

The rollOver anf rollOut actions work fine, but I can't get the getURL to work. In fact, when I click on one of the buttons, the flash movie disappears! I tried attaching the same action to a button that wasn't masked, and it did work.
Is there a problem with masking buttons?
How do I work around this?

Thanks,
Gittit