Problem reaching mc´s in main swf

Hi, I´m having problems with the following…

I have a main swf and an external swf that is loaded onto _level1
In the external swf there is a button with the following code…

Pieza01.onRelease = function(){
_level0._visible = false;
};

Which works fine.
But if i replace it with the following code…

Pieza01.onRelease = function(){
_level0.menu_mc._visible = false;
};

Nothing happens, and I´m sure the instance name “menu_mc” is correct.

Any ideas?
Thanks a lot.