It's a mystery

I’m getting a bit aggrevated with a little problem I have.
Nothing complicated, and it just should work, but I just can’t figure out what I can be doing wrong.

I have a function where I pass a reference to a clip and try to duplicate it.
All very standard stuff that I done 1000’s of times, but this time it just will not work.
This is the code:

function showModifiers(clip:MovieClip):Void{
var cp:MovieClip=clip.duplicateMovieClip("clip_copy",_level0.getNextHighestDepth());

Trace outputs the following:


I know the code is correct and setting up a new blank fla it works.
The function is part of an external class and I tested to just stick it on the root timeline in the .fla with the same trace result.
So I really wonder what could possibly go wrong?
Since the clip variable and depth is there I can’t think of any scope issues that could cause it.
Anyone experienced similar problems and found a cause?