Javascript error on website. How to fix?

When users are browsing to our website.
www.cdb.com.au

Some are recieving a javascript error. Stating the following.

Description: Invalid Argument.
Source: http://www.cdb.com.au/menus/iemenu.js
Line:1

Can only confirm this problem when using IE v8

Do not recieve this error when using firefox, or IE v6 and 7

Any help would be greatly appreciated.