Javascript Read XML from PHP Cannot Read Property

I am having an error with my javascript for the Google Maps API. I have an almost identical script running on another page and it works flawlessly however when this one runs, it writes:

"Uncaught TypeError: Cannot read property 'documentElement' of null"

in the console. I only have one documentElement reference and it is EXACTLY the same as it is in my other page that works correctly. I can't figure out why this one is throwing the error.

My PHP to make the XML:

header("Content-type: text/xml");

// Start XML file, echo parent node
echo '<markers>';
$i = 1;
// Iterate through the rows, printing XML nodes for each
while ($row = @mysqli_fetch_assoc($result)){
  // ADD TO XML DOCUMENT NODE
  echo '<marker ';
  echo 'name="' . parseToXML($row['firstName']) . '" ';
  echo 'address="' . parseToXML($row['address1']) . '" ';
  echo 'city="' . parseToXML($row['city']) . '" ';
  echo 'zip="' . parseToXML($row['zip']) . '" ';
  echo 'comments="' . parseToXML($row['comments']) . '" ';
  echo 'location="' . parseToXML($row['location']) . '" ';
  echo 'lat="' . $row['lat'] . '" ';
  echo 'lng="' . $row['lng'] . '" ';
  echo 'distance="' . $i . '" ';
  echo '/>';
  $i++;
}
echo '<marker name="Test" address="123 Rose Road" city="Austin" zip="78745" comments="" location="" lat="" lng="" distance="home"/>';
// End XML file
echo '</markers>';

My Google Maps API Javascript that errors:

var customIcons = {
      1: {
        icon: 'mapicons/number_1.png',
        shadow: 'http://labs.google.com/ridefinder/images/mm_20_shadow.png'
      }
    };

    function load() {
      var map = new google.maps.Map(document.getElementById("map"), {
        center: new google.maps.LatLng(31.526342, -99.432643),
        zoom: 10,
        mapTypeId: 'roadmap'
      });
      var infoWindow = new google.maps.InfoWindow;


      // Change this depending on the name of your PHP file
      downloadUrl("phpsqlsearch_confirmed.php", function(data) {
        var xml = data.responseXML;
        var markers = xml.documentElement.getElementsByTagName("marker");
        for (var i = 0; i < markers.length; i++) {
          var name = markers[i].getAttribute("name");
          var address = markers[i].getAttribute("address");
          var city = markers[i].getAttribute("city");
          var zip = markers[i].getAttribute("zip");
          var distance = markers[i].getAttribute("distance");
          var comments = markers[i].getAttribute("comments");
          var location = markers[i].getAttribute("location");
          var point = new google.maps.LatLng(
              parseFloat(markers[i].getAttribute("lat")),
              parseFloat(markers[i].getAttribute("lng")));
          var html = "<b>" + name + " - " + location + "</b> <br/>" + address + "<br/><font size=-1 color='blue'>" + comments + "</font>";
          var icon = customIcons[distance] || {};
          var marker = new google.maps.Marker({
            map: map,
            position: point,
            icon: icon.icon,
            shadow: icon.shadow
          });
          bindInfoWindow(marker, map, infoWindow, html);
        }
      });
    }

    function bindInfoWindow(marker, map, infoWindow, html) {
      google.maps.event.addListener(marker, 'click', function() {
        infoWindow.setContent(html);
        infoWindow.open(map, marker);
      });
    }

    function downloadUrl(url, callback) {
      var request = window.ActiveXObject ?
          new ActiveXObject('Microsoft.XMLHTTP') :
          new XMLHttpRequest;

      request.onreadystatechange = function() {
        if (request.readyState == 4) {
          request.onreadystatechange = doNothing;
          callback(request, request.status);
        }
      };

      request.open('GET', url, true);
      request.send(null);
    }

    function doNothing() {}

    //]]>

Any help is greatly appreciated, if you need more information let me know because I am not too familiar with javascript.