$.ajaxSetup --> contentType doesn't work

$.ajaxSetup --> contentType doesn't work

Using: JQuery 1.4.2. 

Creating .Net app. Apparently it errors if i don't set the contentType. So I tried to set it in the ajaxSetup function, but it seems to reset it. The type and dataType attributes seem to work fine, though, just not the contentType.

  1.     $(function () {
  2.         $.ajaxSetup({ type: "POST", contentType: "application/json; charset=utf-8", dataType: 'json' });

  4.         $(".refresh").click(function () {
  5.             $.ajax(
  6.                 {
  7.                     //contentType: "application/json; charset=utf-8",
  8.                     url: 'json.aspx/GetRandomNumber',
  9.                     success: function (data) {
  10.                         $('.result').attr('value', data.d);
  11.                     },
  12.                     error: function (XMLHttpRequest, textStatus, errorThrown) {
  13.                         $('.result').attr('value', XMLHttpRequest + '-' + textStatus + '-' + errorThrown);
  14.                     }
  15.                 }
  16.             );

  18.         });
  19.     });