我尝试使用以下代码在矢量图层上显示标记，

var map;
function init(){
   map = new OpenLayers.Map('map_element',{});
   var wms = new OpenLayers.Layer.WMS(
       'OpenLayers WMS',
       'http://vmap0.tiles.osgeo.org/wms/vmap0',
               {layers: 'basic'},
       {}
       );
       map.addLayer(wms);
       if (!map.getCenter())
       {
           map.zoomToMaxExtent();
       }
       var feature_data = {
           "type": "FeatureCollection",
           "features": [
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [-81, 42]}},
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [-82, 43]}},
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [-80, 41]}},
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [19, -24]}},
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [4, 42]}},
               {"type": "Feature", "properties": {},
                   "geometry": {"type": "Point", "coordinates": [32, 35]}},
           ]
       }
       var format_geojson = new OpenLayers.Format.GeoJSON({});
       var vector_strategies = [new OpenLayers.Strategy.Cluster({distance: 42})];
       vector_layer = new OpenLayers.Layer.Vector('More Advanced VectorLayer', {strategies: vector_strategies});
       var fLayer = vector_layer.addFeatures(format_geojson.read(feature_data));
       map.addLayer(fLayer);
   }

我能够在相应的 div 上看到地图，但缺少标记。在控制台中，我发现以下错误消息，

Uncaught TypeError: Cannot read property 'div' of undefined

我哪里犯了错误？我是 OpenLayers 的新手，如果我犯了任何愚蠢的错误，我深表歉意，我不想使用集群策略，是否必须使用这个，如果不是，我如何跳过集群策略。？