Why do masks kill dynamic btns?

I can't see why this would happen but the btns just simply won't appear when I add a mask to my stage...

Simple call from document class:

btns = new ButtonSet();

btns.addButtons([restaurant_btn, owner_btn, chef_btn]);

addChild(btns);

I have a mask on the stage, if I remove the mask, the buttons show, if I put the mask back, the buttons go away....

Anyone know why?

Thanks in advance