got JavaScript error on Create Item page

Could someone help?

I think GoPlanit is a really wonderful website.

However, unfortunately I cannot create new item due to the JavaScript error.
I got JS error when clicking on the "Submit" button on the "Creat Item" page.
Here below is the detail of the error which was displayed by IE.

--quote--
Webpage error details

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; GTB6; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; OfficeLiveConnector.1.4; OfficeLivePatch.0.0; .NET CLR 1.1.4322; .NET CLR 3.0.30618; .NET CLR 3.5.30729)
Timestamp: Tue, 8 Sep 2009 13:59:15 UTC

Message: 'undefined' is null or not an object
Line: 1
Char: 4114
Code: 0
URI: http://www.goplanit.com/js/base.js?2472
--unquote--

Thanks in advance for your help.