is assert_select indicated for XML?

Railsers:

I'm playing with the Beast, and this test fails:

   def test_should_show_topic_as_rss
     get :show, :forum_id => forums(:rails).id, :id => topics(:pdi).id,
              :format => 'rss'
     assert_response :success
     assert_select 'channel'
   end

It emits "ignoring attempt to close channel with link".

So why are the Beastie boys testing XML with assert_select? Wouldn't an assertion that didn't expect HTML work better there?