MOSS 2007 RSS Viewer's XSL Link not working

Has anybody been able to get the MOSS 2007 RSS Viewer Web Part to work
with the XSL Link?

I have an RSS feed file and a style sheet in the same anonymously
accessible location. It works fine if I just point the web part at
the RSS feed. If I try to use the XSL link, it fails every time with
this error in the logs:

RssWebPart: Exception handed to HandleXslException.HandleException
System.ArgumentNullException: Value cannot be null. Parameter name: s
at System.IO.StringReader..ctor(String s) at
Microsoft.SharePoint.WebPartPages.DataFormWebPart.GetXslCompiledTransform()
at
Microsoft.SharePoint.WebPartPages.DataFormWebPart.PrepareAndPerformTransform()

I started by using the default XSL copied from "XSL Editor..." in the
web part.

It still doesn't work even when I use this for the XSL file:

xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
FOO


(reposted after waiting 24 hours after setting up an alias)