[jQuery] superfish menu, Error: $("ul.sf-menu") is null

[oconshaw]

I'm using the superfish menu with a navigation module in dotnetnuke
that emits the page structure as an unordered list.  Most pages work
fine with the menu system and superfish.  However, some pages give me
the following error: Error: $("ul.sf-menu") is null, source
file:http://blahblahblah.com/default.aspx line: 4

The menu still works using the CSS, but the superfish features fail at
that point.  Any ideas on what is wrong and how to fix?  Thanks.