hjælp til ajax

Hej eksperter jeg har problemer med nedenstående kode:

function getHTTPObject(){
        if (window.ActiveXObject) return new ActiveXObject("Microsoft.XMLHTTP");
        else if (window.XMLHttpRequest) return new XMLHttpRequest();
        else {
            alert("Your browser does not support AJAX.");
            return null;
        }
    }

    function setOutput(elementId){
      alert(httpObject.readyState);
      if(httpObject.readyState == 4){
        document.getElementById(elementId).innerHTML = httpObject.responseText;
      }
    }

    function countryHasBeenChanged(){

        document.getElementById('county_status_select').options[0].selected = true;
        document.getElementById('county_status_select').disabled = true;
        document.getElementById('city_status_select').options[0].selected = true;
        document.getElementById('city_status_select').disabled = true;
        document.getElementById('state_status_select').options[0].selected = true;
        document.getElementById('state_status_select').disabled = true;
        document.getElementById('address_field').value = '';
        document.getElementById('address_field').disabled = true;

        if(document.getElementById('country_status_select').value > 0){
            httpObject = getHTTPObject();
            if (httpObject != null) {
                httpObject.open("GET", "AjaxHelper.php?country=" + document.getElementById('country_status_select').value, true);
                httpObject.send(null);
                httpObject.onreadystatechange = setOutput('state_div');
            }
        }
    }

    function stateHasBeenChanged(){
        document.getElementById('city_status_select').options[0].selected = true;
        document.getElementById('city_status_select').disabled = true;
        document.getElementById('county_status_select').options[0].selected = true;
        document.getElementById('county_status_select').disabled = true;
        document.getElementById('address_field').value = '';
        document.getElementById('address_field').disabled = true;

        if(document.getElementById('state_status_select').value > 0){

            httpObject = getHTTPObject();
            if (httpObject != null) {
                httpObject.open("GET", "AjaxHelper.php?state=" + document.getElementById('state_status_select').value, true);
                httpObject.send(null);
                httpObject.onreadystatechange = setOutput('county_div');
          }
        }
    }


    function countyHasBeenChanged(){
      document.getElementById('city_status_select').options[0].selected = true;
      document.getElementById('city_status_select').disabled = true;
      document.getElementById('address_field').value = '';
      document.getElementById('address_field').disabled = true;

      if(document.getElementById('county_status_select').value > 0){
            httpObject = getHTTPObject();
            if (httpObject != null) {
                httpObject.open("GET", "AjaxHelper.php?county=" + document.getElementById('county_status_select').value, true);
                httpObject.send(null);
                httpObject.onreadystatechange = setOutput('city_div');
            }
      }
  }

  function cityHasChanged(){
    if(document.getElementById('city_status_select').value > 0){
      document.getElementById('address_field').disabled = false;
    }else{
      document.getElementById('address_field').value = '';
      document.getElementById('address_field').disabled = true
    }
  }

  var httpObject = null;



Problemet opstår når jeg fjerner "alert(httpObject.readyState);", derefter insættes der ikke værdier i min html.