"；值不在预期范围内"；Window 8 ObservableDictionary错误

在异步方法中将值设置为DefaultViewModel时出错

    async void SpotlightVideo_FadeIn_Completed(object sender, object e)
    {
        this.DefaultViewModel["SpotlightImageSource"] = (await ExtensionMethods.GetSingleImage(1, SpotlightVideo.Image)).Source;
        Spotlight_Videos s = this.DefaultViewModel["SpotlightVideos"] as Spotlight_Videos;
        //s.Discription = SpotlightVideo.Discription;
        //s.Image = SpotlightVideo.Image;
        //s.Video_Id = SpotlightVideo.Video_Id;
        //s.Video_Name = SpotlightVideo.Video_Name;
        //s.VideoPath = SpotlightVideo.VideoPath;
        this.DefaultViewModel["SpotlightVideos"] = SpotlightVideo;
        //s = SpotlightVideo;
        SpotlightVideo_FadeOut.Begin();
    }

mehtod中的第一行运行良好DefaultViewModel["SpotlightImageSource"]但this.DefaultViewModel["SpotlightVideos"] = SpotlightVideo;给我的错误为：

"值不在预期范围内。"

但当我为各个属性设置值时，它运行良好，没有错误。

当我将整个对象传递给s变量时，它运行时没有任何错误，但并没有给出预期的结果。

我调试了代码，基本上错误是从ObservableDictionary.cs 生成的

private void InvokeMapChanged(CollectionChange change, string key)
    {
        var eventHandler = MapChanged;
        if (eventHandler != null)
        {
            eventHandler(this, new ObservableDictionaryChangedEventArgs(change, key));
        }
    }

错误行为：

eventHandler(this, new ObservableDictionaryChangedEventArgs(change, key)); 

请帮帮我。