IE7/Vista + Request().send(form.toQueryString()) = FAIL

[kfancy]

I'm encountering the dreaded "Object doesn't support this property or
method" error on IE7 / Vista, while trying to submit form data via a
Request object.

This works fine, but I have to jump through the hoops and collect my
own data (please note that all variables are properly initialized for
passing via the data:{} object):

var ajrq = new Request({
        url:"handler.php",
        onSuccess: function(responseText, responseXML) {
                update_page(responseText);
        },
        onFailure: function() { alert("Request failed."); }
}).send({data:{ var1:var1_txt, var2:var2_txt, var3:var3_txt,
var4:var4_txt }});

However, if I change my send() line to use the built-in toQueryString,
then IE throws an error:

var ajrq = new Request({
        url:"handler.php",
        onSuccess: function(responseText, responseXML) {
                update_page(responseText);
        },
        onFailure: function() { alert("Request failed."); }
}).send(myformobject.toQueryString());

myformobject is a reference to my HTML form, passed via this.form
object from the submit button click event.

My form doesn't have anything tricky, just the standard input types
(text, textarea, select, hidden, checkbox).

Has anybody else encountered this problem, and/or have a solution?