[mx] Adding mc's in an emptyMovieClip fails

Fairly simple question:

When I create a new mc:

 _root.createEmptyMovieClip("container", 1);

and try to attach one into it:

 _root.container.attachMovie("exc","child_a", 1);

or simply:

 _root.container.createEmptyMovieClip("child_b", 1);

nothing works - it simply doesn’t want to get in…

Even when I give the “container” content with [.lineTo()], nothing continious to happen.

I guess it has something to do with the ‘emptyness’ of the mc, but how to solve?