slideToggle() method (jquery 1.4.2) and Prototype (Array length must be assigned a finite positive number)

slideToggle() method (jquery 1.4.2) and Prototype (Array length must be assigned a finite positive number)

I hope someone can shed some on light on the following issue.
 
I have a page which includes the Prototype library.  Also on this page is a collapsible menu, which comes from http://www.lousyllama.com/sandbox/collapsible-menu. The first menu at left is in use. I had to modify the original jQuery code to get the collapsible menu to work on jQuery 1.4.2.
 
Problem:
 
An exception is being thrown (by the jQuery framework?):
 
Line: 464
Error: Array length must be assigned a finite positive number
 
The HTML:
 
<
ul class ="example_menu" id ="rd_menu_sample">
< li >< a class ="collapsed" href ="/books.aspx"> Books </ a >< ul >
< li >< a title ="Kids's Books" href ="/Kids.aspx"> Kids </ a ></ li >
< li >< a title ="Adult Books" href ="/Adult-Books.aspx"> Adults </ a ></ li >
< li >< a title ="Audio Books" href ="/Audio-Books.aspx"> Audio </ a ></ li ></ ul > </ li >
< li >< a class ="collapsed" href ="/Toys.aspx"> Toys </ a >< ul >
</ ul >
 
The jQuery code:
 
By removing slideToggle(), the exception is not thrown.
 
jQuery(document).ready(function() {
    // Toggle the selected menu's class and expand or collapse the menu
    jQuery("#rd_menu_sample > li > a").mouseover(function() {
        jQuery(this).toggleClass("expanded").find("+ ul").slideToggle("medium");
    });



});
 
In the HEAD tag, in order shown:
 
<script src='http://IP address/xxx/Themes/Print Book/scripts/jquery-1.4.2.min.js' language='javascript' type='text/javascript'></script>
<script src='http://IP address/xxx/Themes/Print Book/scripts/CollapsibleMenuNoConflict.js' language='javascript' type='text/javascript'></script>
 
        <script language="javascript" type="text/javascript">
            jQuery.noConflict();
        </script>




 
<script language='javascript' src='http://IP address//xxx/SmartView/lightbox/js/prototype.js'></script>
<script language='javascript' src='http://IP address//xxx/SmartView/lightbox/js/lightbox.js'></script>
<script language='javascript' src='http://IP address//xxx/SmartView/lightbox/js/scriptaculous.js?load=effects'></script>

 
What exactly is the problem and can it be resolved?
 
Thanks in advance.