Leaf in Internet Explorer

My site works perfectly in Chrome, but when I check it using Internet Explorer, I get the following pop-up box, which appears on every page, and I have to press OK to get rid of it:

Message from webpage:

element does not exist in DOM!

See http://www.sebastars.co.uk/

The person who highlighted the issue uses IE all the time, and doesn’t have the same problem on any other site.

Thanks in advance for any advice.