[Plugin: upPrev] ReferenceError: Can't find variable

I have included the upPrev js into my combined js file and it works perfectly — UNLESS I visit the homepage or any other page which is not specifically a post or page. Here’s the error I see in the console:

ReferenceError: Can't find variable: iworks_upprev

JS is not my forte beyond a certain point so I’m at a loss as to how to supress this error message. Here are a few links where the error can be seen:

http://www.psdcovers.com/
http://www.psdcovers.com/category/psd/graphics/

http://ww.wp.xz.cn/extend/plugins/upprev/