Uncaught TypeError "has no method 'getLatLng'"

Question

I have been experiencing this error for some time (as seen in Google Chrome JS Error Console):

Uncaught TypeError: Object function (value){ var i; for (i=0; i < this.length; i++) { if (this[i] == value){ return true; } } return false; } has no method 'getLatLng'

It doesn't keep the page from loading and doesn't seem to affect the performance, but it is an error so I want to fix it.

Here's the code from that page:

window.onload=load;

var isEditing = false;
var polygon = null;
var map = null;
var isCompatible = GBrowserIsCompatible();
var markers = new Array();
var iconIncluded = MapIconMaker.createLabeledMarkerIcon({addStar: false,label:"", primaryColor: "#006600"});
var iconExcluded = MapIconMaker.createLabeledMarkerIcon({addStar: false,label:"", primaryColor: "#CCCCCC"});

    function editClick() {
            isEditing = !isEditing;

            if(isEditing && polygon != null) {
                polygon.enableDrawing();
            } else {
                polygon.disableEditing();
            }
    }

    // Update all markers with in poly status
    function updatePoints() {
        mapvoters = new Array();
        for(var i in markers) {
            var marker = markers[i];
            var point = marker.getLatLng();
            var inPoly = polygon.containsLatLng(point);
            var voterid = marker.voterid;

            if(inPoly) {
                marker.setImage(iconIncluded.image);
                Array.prototype.inArray = function (value){
                    var i;
                    for (i=0; i < this.length; i++) {
                        if (this[i] == value){
                        return true;
                        }
                    }
                    return false;
                };

                if(mapvoters.inArray(voterid)){
                }else{
                    mapvoters.push(voterid)
                }
                $('#SelectedVoters').attr('value', String(mapvoters));
                $('#NumSelected').html(mapvoters.length);
                //$('#PolyIDs').html(String(mapvoters));
            }else{
                marker.setImage(iconExcluded.image);
            }
        }

    }

    function resetPolygon() {
        if(polygon != null) {
            map.removeOverlay(polygon);
        }

        polygon = new GPolygon([], "#000000", 1, 1, "#336699", 0.3);
        map.addOverlay(polygon);

        GEvent.addListener(polygon, "endline", function() {
            setTimeout(editClick, 50);
            setTimeout(updatePoints, 50);
            $('#map').parents('#page_layout').find("#PolygonHelp").slideToggle(500);
            GEvent.addListener(polygon, "lineupdated", function() {
                setTimeout(updatePoints, 50);
            });
        });

    }

//<![CDATA[
function load() {
    if (isCompatible) {
        // Create Map
        map = new GMap2(document.getElementById("map"));
        map.setCenter(new GLatLng(40, -90), 3);

        // Add controls
        map.addControl(new GLargeMapControl());
        map.addControl(new GMapTypeControl());

        resetPolygon();

       GDownloadUrl("<?php echo $XML; ?>", function(data) {
          var xml = GXml.parse(data);
          var markers_xml = xml.documentElement.getElementsByTagName("marker");
          var bounds = new GLatLngBounds();
          for (var i = 0; i < markers_xml.length; i++) {
            var listid = markers_xml[i].getAttribute("lid");
            var voterid = markers_xml[i].getAttribute("voterid");
            var contacted = markers_xml[i].getAttribute("contacted");
            var name = markers_xml[i].getAttribute("name");
            var address = markers_xml[i].getAttribute("address");
            var type = markers_xml[i].getAttribute("type");
            var iconcolor = markers_xml[i].getAttribute("iconcolor");
            var point = new GLatLng(parseFloat(markers_xml[i].getAttribute("lat")),
                                    parseFloat(markers_xml[i].getAttribute("lng")));
            if(contacted == '2'){
                var thecolor = "#CCCCCC"
            }else{
                var thecolor = iconcolor
            }

            var marker = createMarker(point, voterid, name, address, type, thecolor, listid);
            map.addOverlay(marker);
            bounds.extend(point);
            markers.push(marker);
            markers[i].voterid = voterid;
            markers[i].contacted = contacted;
          }
          map.setZoom(map.getBoundsZoomLevel(bounds));
          map.setCenter(bounds.getCenter()); 
        });
        updatePoints();

    }
}

function createMarker(point, voterid, name, address, type, color, listid) {
  var newIcon = MapIconMaker.createLabeledMarkerIcon({addStar: false,label:"", primaryColor: color});
  var marker = new GMarker(point,{ icon: newIcon, title: name } );
  var html = "<b>" + name + "</b> <br/>" + address + "<br/>" + voterid;
  GEvent.addListener(marker, 'click', function() {
    marker.openInfoWindowHtml(html);
  });
  return marker;
}

function addToWalkList(voterid){
    Array.prototype.inArray = function (value){
        var i;
        for (i=0; i < this.length; i++) {
            if (this[i] == value){
            return true;
            }
        }
        return false;
    };

    if(mapvoters.inArray(voterid)){
    }else{
        mapvoters.push(voterid)
    }
}

function clearMarkers(){
    $('#NumSelected').html("0");
    resetPolygon();
    for(var i in markers) {
        var marker = markers[i];
        marker.setImage(iconExcluded.image);
    }
    document.getElementById('edit_button').disabled = '';
    editClick();
}

//]]>
</script>

I think I must be initiating the script wrongly...I'm really not sure. After several hours of research and tinkering, I turn to this community for help. Point me in the right direction. All help needed and appreciated. Thanks!