I'm getting a javascript error.

URL: http://bit.ly/dfjvmT

It has to do with this section of code:

/*
 * Activate jQuery
 */
$(document).ready(function() {          

/*
 * Show hidden field when 'Other' selected from dropdown
 */
$.listHidden = { 
    '6' : $('.referrer_other')
};
$('#referrer_select').change(function() { 
    // hide all
    $.each($.listHidden, function() { 
        this.hide(); 
    });
    // show current
    $.listHidden[$(this).val()].slideDown(250);
}).change();    

});

The code works like this. The form has a dropdown asking "How did you find us?" If the user selects 'other', which is an <option> with value=6, a text input field .referrer_other becomes visible.

Javascript is currently reporting that: '$.listHidden[...] is null or not an object'. It's an error that appears in IE. I found this code on the internet somewhere. Is there a way to fix it, or should I look for something else?

Thanks