onmousewheel isn't captured

Hi all,
I'm working on a navigation system which use the mouse wheel as a mean to move through a 3d space. Anyway, the problem is that it seems that the mouse wheel isnt working until the flash is clicked at first. You can test this with this simple script on the first frame of a movie :

var ml:Object=new Object();
ml.onMouseWheel=function(delta){trace(delta)}
Mouse.addListener(ml);

It won't trace anything until you click on the stage. Is this a known bug, is there any workaroung ?
Thanks a lot,
rb