TypeError: Error #1009 when adding a marker

[rock]

hi I am trying to develop a navigational system with googles api for
flash and am at the point of adding some markers. I'm quite new to
action script and don't know exactly how to resolve this error

TypeError: Error #1009: Cannot access a property or method of a null
object reference.
        at com.google.maps.overlays::Marker()


the map code I am working with at the moment is

//--Google Map --//

import com.google.maps.Map;
    import com.google.maps.LatLng;
    import com.google.maps.MapEvent;
    import com.google.maps.MapType;
    import com.google.maps.controls.MapTypeControl;
    import com.google.maps.controls.ZoomControl;
    import com.google.maps.controls.PositionControl;
        import com.google.maps.overlays.MarkerOptions;
    import com.google.maps.overlays.Marker;





var map:Map = new Map();
map.key =
"ABQIAAAA4mtg0_yOAQswdZC6f0NGKxS1jE4jbpVLla8ckf7FZdRsusTb7hRHWx299NvLgk13X0-
T0Vk9xZW7vg";

map.addEventListener(MapEvent.MAP_READY, onMapReady);
this.addChild(map);

//--Map Size  / x-y position --//
map.setSize(new Point(435, 435));
map.x = 340;
map.y = 125;



//--Map loading Position and Map Type options --//
function onMapReady(event:Event):void {
  map.setCenter(new LatLng(51.54676205809519,-0.011587142944335938),
14, MapType.NORMAL_MAP_TYPE); map.removeMapType
(MapType.HYBRID_MAP_TYPE); map.removeMapType
(MapType.SATELLITE_MAP_TYPE);

  //--Declaring map controls--\\
        map.addControl(new ZoomControl());
        map.addControl(new PositionControl());
        map.addControl(new MapTypeControl());
    }

          //--Map Markers--\\

    var marker:Marker = new Marker
    (new LatLng
(51.54676205809519,-0.011587142944335938));
    map.addOverlay(marker);


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