Dynamic id selector. - value is NULL

Dynamic id selector. - value is NULL

Hello i`m trying to make a function that show and hide some elements but i have problem with select ID`s from div`s.

This is my code:

HTML:
  1. onmouseover="change_visibility(document.getElementById(this.id));"
JS:

  1. function change_visibility(idd){
        var element = '#menu_'+idd.id;
        //alert(element);
        $(element).hover(function () {
            if ($(this).is(":hidden")) {
                $(this).show();
            } else {
                $(this).hide();
            }
        });    
    }










Alert works fine but function says "$(element) is NULL", I have no ideas to solve this.
Anybody know what I'm doing wrong?