Why do I get an "Error on Page" every time I use ShrinkTweet?

Every time I try to use TweetShrink, the page loads fine, but as soon as I type anything in the top box or copy/paste into the top box, the page all of a sudden shows an "Error on Page" notice down in the lower left hand corner of my web browser. I refresh, it still happens. The "error" seems to disable the "Shrink Tweet" button between the two text boxes, as I can't use it.

I have tried to use the site many times in the last few months, always with the same problems.