Define hit area for animated button

Hello, I’ve created an animated button where when you rollover it - a message box appears. My problem is that the button hit area is the whole size of the movie clip (including the message box), so the box will appear even if you haven’t rolled over the button.

So basically I want to define the hit area to just the size of the button itself, rather than the whole movie clip. I’ve searched for a solution and tried a couple of things without success, so any help would be much appreciated.

Thanks!