SharePoint Lookup Field 20 Item JS error

Question

Hi Everyone,

I've got an issue with SharePoint Server 2007 SP1 which seems to be documented in various forms, but I haven't been able to find an answer to my seemingly simpler question.

http://social.msdn.microsoft.com/Forums/en-US/sharepointcustomization/thread/040533d2-c738-4ac2-b2d6-65a1602fa2d1

Essentially, we have a form with a lookup field to another SharePoint list. The lookup field has more than 20 items, and so SharePoint changes the type/behaviour of the field as per the standard behaviour. However, with my custom skin applied, clicking on this drop down causes a JS error from within the Core.js file, as described here:

http://splitnut.blogspot.com/2009/06/lookup-fields-in-moss-javascript-error.html

What I haven't been able to figure out is why this is only an issue when my custom master page is applied, and not the OOTB master pages. I've been through and tried to see what might cause this, but haven't been able to track down the cause of the behaviour.

Any help would be much appreciated.

Many thanks.