Although the argument may at first sound correct, it is not valid.  In general, from “I can doubt the existence of X; I cannot doubt the existence of Y,” we cannot infer that X and Y are distinct.  Compare:

         1.  I can doubt that the winner of the 1999 All-Ohio Cross-Dressing Contest exists.
           2.  I cannot doubt that my daughter exists.  [Say, because she is standing right in front of me and we are chatting.]
          _________________________________________________

           3.  My daughter is not the winner of the 1999 All-Ohio Cross-Dressing Contest.   [1,2]

That conclusion clearly does not follow.  I can doubt the one and not the other so long as I don’t know that my daughter is the actual winner of the contest; all that  is perfectly consistent with her actually being the winner.  (Don’t be distracted by the fact that on Dream / Evil Genius grounds, Descartes would not grant the truth of premise (2).  The point is that, whether or not (2) is true, (3) does not follow from (1) and (2).)

    For an explanation of this, see the optional handout, "Descartes' First Argument and Leibniz' Law."