[jQuery] 1.4 issues using .is()

I have this that has worked just fine in 1.3.2:

if($(this).is('li')){
...do something...
};

In 1.4 (and 1.4.1), however, that produces an error:

"g.nodeName is undefined"

The only mention I found of this is here:

http://forum.jquery.com/topic/window-is-div-returns-error-jquery-1-4

and that's just a mention of the problem.

Has anyone else run into this and maybe have a theory as to what is happening?

-Darrel