It has been reported by several users that you will get this error if you are using certain browser versions. What is the version of your OS and browser?
It looks like it was reported as software (Firefox) bug by some other user also. Most of those Javascript bugs are related to browser software, not the website code used. The fact that it works properly in Chrome shows that the culprit is the browser? The funny thing is, some users say it works fine in Firefox (Windows and Mac). I'm wondering if anyone else using Firefox 17 has the same issue like you.