iframe is not created in IE for Nice Menu

i am using Nice menu, it works perfectly with FF. Of course IE has issue. My sub-menu are covered by Navigation block below. So I used Nice_menu.js to fix the IE bug, but it is not working. In IE, everytime when I hover to the menu, I get "Error on page" at the left corner, the detail says "the object doesn't support the property or method", and I don't know which object is.

can anyone give me some hint? many thanks