An error on converting MathML to LaTeX

While using Dr. Carlisle's stylesheet mentioned here to Convert MathML to LaTeX, I am getting the following error:
"The specified node cannot be inserted as the valid child of this node, because the specified node is the wrong type"
My XML file has following content:
<?xml version="1.0"?>
<math xmlns:mml="http://www.w3.org/1998/Math/MathML">
  <msup>
    <mi>a</mi>
    <mn>2</mn>
  </msup>
</math>
I think, I may be missing something simple. Please help.
Thanks,Saf