But it works with this file
<?xml version="1.0" encoding='utf-8'?>
<feed xmlns="http://example.org"> <title>Projects</title> </feed>
...indeed it shouldn't work, as Michael already pointed out. But you are lucky; this issue has recently been fixed due to a bug report from Andy Bunce, which means that it should work again with the latest snapshot.
Btw, if you know that there's only one namespace in your document, you may as well use the wildcard instead of fully writing down your namespace:
//*:feed
C.