Multilevel sortable problem in IE

Multilevel sortable problem in IE

Hello,

I've got a question about using sortable in a multilevel list.

In a CMS I'm using jQuery UI sortable to order my menu items. The system works in all browsers, but in IE it won't work. I've made a standalone version where you can see the problem: http://hhwcms.nl/temp/
When you try to order a sub menu you get the problem, the parent item will follow your mouse. In Firefox I can just drag the sub item, but in Internet Explorer the parent will follow.
I've posted my source code on pastebin: http://pastebin.com/GE7s10GU

Hopefully could somebody help me.

Thanks in advance!