Using Request.JSON with urlEncoded: false

[websam]

I'm calling a .net webmehod using MooTools and it works fine, as long
as I do not send data with. As soon as I add a key:value pair it fails
to load the webmethod :

var showRoles = new Request.JSON({
    url: 'default.aspx/getRoles',
    method: 'post',
    evalResponse: true,
    urlEncoded: false,
    headers: { 'Content-Type': 'application/json; charset=utf-8' },
    data: {'id': '10'},
    onSuccess: function(json) {
        loadRoles(json.d); //--- Method that generates a table.
    },
}).send();

urlEncoded must be set to false and headers set to { 'Content-Type':
'application/json; charset=utf-8' }

When i add the data: { 'id':'10' } it shows up in firebug as id=10
instead of id 10 as supposed when I use post and not get.

Can some one help me on this matter ?