Anscombe + Descartes

If Descartes' argument for dualism really is (as Anscombe seems to be suggesting in her essay on "The First Person"):

I know there's a thinking thing (namely myself).
I don't know whether there are any bodies.
Therefore: there's a thinking thing which is not a body.

then it seems to me, his argument is immediately fallacious. I mean, it's exactly like someone saying, after shaking a box and hearing a rattle:

I know there's a thing-inside-this-box (namely the one I heard rattle).
I don't know whether there are any marbles-inside-this-box.
Therefore: there's a thing in this box that isn't a marble.

Given that objects can have multiple different properties A and B, it's obviously possible to know that there's something with property A while being ignorant as to whether there's anything that has property B!

So there's no need for Anscombe to go to the lengths of denying that "I" refers to escape the force of such a weak argument as this, it seems to me.