“/” becomes “undefined” in comments

I’ve noticed recently that anytime someone types a slash character (/) in a comment on my site, WordPress changes it to the string “undefined”. (See the first sentence in the comment I’ve linked.)

Any idea why this is happening? I’ve been fixing these manually in prior comments, but it seems like something that shouldn’t be happening in the first place.

The page I need help with: [log in to see the link]