Tiny issue with a layer

Hi,

I'm currently trying to do the same copy to clipboard trick Muppet Mark-QAl63s showed me in this thread: http://forums.adobe.com/thread/840440?tstart=0

Unfortunately with my current document I get an error when trying to execute this:

var lineRef = app.activeDocument.pathItems.add();

and there error is: "Target layer cannot be modified"

....which is a bit cryptic to me. Why can't the layer be modified ?

I've printed to the console app.activeDocument.activeLayer.locked and it prints false.

What does that error mean and how can I get past it ?

Thanks in advance,

George