Enabling Nice Primary Menus disables fieldset expansion

Just to let you know
I experienced the same issue last night. My only option, unfortunately was to disable the module.

I reproduced the problem with a new, pretty bare-bones toy site running locally on a Mac with MAMP. Simply turning on Nice Primary Menus causes fieldset headers to act like plain text, without an expansion triangle to the left and with no response to mouse clicks. This holds true under both Firefox and Safari and the plain-vanilla Garland theme. (For the record I didn't edit the theme or any menus.)

Turning off Javascript in the browser and reloading the page effectively expands all the fieldsets, so this may be a temporary work-around for some purposes. A search of the Drupal site turns up numerous problems with fieldset expansion so this may be a symptom of a broader problem.

I accidentally fixed this problem trying to solve a separate "page loaded with errors" warning in IE ("object expected / code 0").

I noticed in FF's error console that the error was "'active_mid' not defined". I add the following line near the top of "nice_primary_menus.js" and it fixed both problems:

function active_mid() {}