Sorry, my fault.
You also need to pass a new XmlUrlResolver to the transform method as you
are using the document function.
myXslTransform.Transform(myXPathDocument, null, writer, new
XmlUrlResolver());

Joe
http://joe.fawcett.name

I presume, Joe has given you the right fix to your code. So, you are
most likely invoking the transformation wrongly, which Joe's
workaround should solve.