Ajax request fails in internet explorer

[jamest]

Could someone help me figure out why this AJAX request is not completed in Internet Explorer. Thanks.

function openmanagegraphs() {
  document.getElementById('managegraphs').style.display = 'block';
  var xmlhttp;
  if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest();
  }
  else { // code for IE6, IE5
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  xmlhttp.onreadystatechange = function () {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
      document.getElementById("results").innerHTML = xmlhttp.responseText;
    }
  }
  xmlhttp.open("POST", "PHP/mgon.php", true);
  xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  CurrentRun;
  xmlhttp.send("mgon=true");
}