SOS,求助在WebClient中使用Newtonsoft.Json解析Json出错的BUG

Question

        public MainPage()
        {
            InitializeComponent();

            WebClient web = new WebClient();
            web.DownloadStringCompleted += new DownloadStringCompletedEventHandler(web_DownloadStringCompleted);
            web.DownloadStringAsync(new Uri("http://localhost/list.json"));
        }

        void web_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            try
            {
                string sJson = e.Result;
                var p = JsonConvert.DeserializeObject(sJson, typeof(OpernItem));
                OpernItem oi = p as OpernItem;
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }

代码如上，sJson字符串已经获得值了也没问题，为什么反序列化就出错了，而且最诡异的是，用控制台调试是好的。
 ex.Message:
Could not load type 'System.Dynamic.IDynamicMetaObjectProvider' from assembly 'System.Core, Version=3.7.0.0, Culture=neutral, PublicKeyToken=969DB8053D3322AC'.
要疯了！