Strange query string issue with IIS 5.0

I am having a strange issue trying to get my ATOM feed working.

http://www.myblog.com/?feed=rss2 works
http://www.myblog.com/?feed=atom returns a blank page

http://www.myblog.com/index.php?feed=atom works
http://www.myblog.com/wp-atom.php works

Again, I am running PHP 5.2.5 on IIS 5.0 w/ ISAPI

Thanks for any ideas.