必应Bing地图Silverlight控件自定义图钉

我尝试为Bing地图Silverlight控件制作自定义图钉,但只能添加1个图钉。在第二个图钉处,我得到以下错误:

System.ArgumentException: Value does not fall within the expected range.
   at MS.Internal.XcpImports.CheckHResult(UInt32 hr)
   at MS.Internal.XcpImports.Collection_AddValue[T](PresentationFrameworkCollection`1 collection, CValue value)
   at MS.Internal.XcpImports.Collection_AddDependencyObject[T](PresentationFrameworkCollection`1 collection, DependencyObject value)
   at System.Windows.PresentationFrameworkCollection`1.AddDependencyObject(DependencyObject value)
   at System.Windows.Controls.UIElementCollection.AddInternal(UIElement value)
   at System.Windows.PresentationFrameworkCollection`1.Add(T value)
   at MapInfo.Silverlight.CitiesControl.MainPage.c_GetCitiesCompleted(Object sender, GetCitiesCompletedEventArgs e)

有人知道我可能做错了什么吗? 在将其添加到地图之前,我正在设置以下属性:

    public Location Location
    {
        get
        {
            return this.GetValue(MapLayer.PositionProperty) as Location;
        }
        set
        {
            this.SetValue(MapLayer.PositionProperty, value);
        }
    }
   this.SetValue(MapLayer.PositionOriginProperty, PositionOrigin.BottomLeft);