update to prototype 1.6.1 causing Error code 12 with select() function

Question

Ever since we upgraded to prototype 1.6.1, I have been getting errors when using the select function. the error is... An invalid or illegal string was specified" code: "12 It stops at line 3299 in prototype.js, which is: results = $A(root.querySelectorAll(e)).map(Element.extend);

Example of my code where I call the function...

variables used (and their values according to firebug watch): elem = div#addressForm[0].addressForm index = [0] [id='AddressIsService" + index +"'] amounts to [id='AddressIsService[0]'] what i am checking is an radio input by that id - AddressIsService[0]

in the code...

    if(elem.select("[id='AddressIsService" + index +"']")[0].checked)
       {
               elem.select("[id='cityFieldRequired" + index +"']")[0].setStyle({display:'none'});
       }

If I downgrade to previous prototype version, it works. Anyone have a clue???