JS Error in IE7 in nice_menus.js

While loading any page - an JS error "'null' is null or not an object"

in nice_menus.js in function

function IEHoverPseudo() {
$("ul.nice-menu li.menuparent").hover(function(){
$(this).addClass("over").find("> ul").slideDown("fast").addShim();
},function(){
$(this).removeClass("over").find("> ul").removeShim().slideUp("fast");
}
);

As I understand it is due to interaction with some another script ?...
}