Intermittent Javascript MRAP error

I have a for loop which moves all elements of an "Artwork" layer to a "working" layer so that I can edit the elemets without a chance of affecting the original layer:

for (var i=app.documents[0].layers['Artwork'].pageItems.length-1; i>=0; i--)

{

          try

          {

                    app.documents[0].layers['Artwork'].pageItems[r].duplicate(app.documents[0].layers['workin g'], ElementPlacement.PLACEATEND);

          }

          catch(e)

          {

                    alert(e);

          }

}

This seems to work fine for three or four documents, and then begins to return the following error:

Error: an Illustrator error occurred: 1346458189 ('MRAP')

Action on every document then returns this error, even ones on which it has previously worked. The only solution is to quit Illustrator and relaunch.

Am I missing something obvious?

Thanks in advance.