Mask + cacheAsBitmap problem

hello
i was wondering if anybody could help me out with a strange issue

usually when i want to make a feathered/gradient/blurred mask, i set the mask and the masked object’s .cacheAsBitmap to true, and everything works.

recently i seem to have lost my touch, since, after doing the same thing the mask simply stopped working. it’s still invisible, but the mc that should be masked isnt.

as soon as i remove cacheAsBitmap everything works without the gradient, add cacheAsBitmap - everything stops working again. ?

syntax i use is

var emask:Shape = new Shape();
emask.graphics.beginFill(0);
emask.graphics.drawRect(0,0, 200, 200);
emask.graphics.endFillI();
emask.filters = [new BlurFilter(5,5,3)];

var mc:MovieClip = new LibraryMc();

mc.cacheAsBitmap = true;
emask.cacheAsBitmap = true;
mc.mask = emask;

addChild(mc);
addChild(emask);

i have reordered it many times to see if something changes, nothing.

anybody got any idea?
thanks,
michael