You sir, are a genius!
That wasn’t quite it, but you led me in the right direction. The problem was, I’m using custom post types on my site; articles and projects—there are no “posts”.
When WordPress sees there are no posts, it looks like it just throws this error. All I needed to do was change the URL to this: feed://51bits.com/feed/?post_type=articles
Bingo.
Thanks mate, that was all I needed: a slap in the face!