5D Mk III: Outline of Dark Ring in ViewFinder

I own a Canon 5D Mk III. I have had it for a couple of weeks and am still within the 30-day return window.

When I look through the viewfinder at bright scenes like the sky, I see a very noticeable outline of a dark ring. The top and bottom of the ring disappear beyond the top and bottom of the viewfinder: the ring is larger than the viewfinder area.

As far as I can tell, the ring outline does not appear on my photographs.

It appears regardless of which lens I use (and I have been using L series lenses exclusively so far).

I've tried Googling the issue but haven't found anything. So I thought I'd ask the learned, passionate 5D owners here.

Thanks a bunch to anyone who can shed any light on this for me.