thrown TargetInvocationException after calling sentiment API

We are developing Silverlight web application to display the tweets on the map and analyse the tweets by using sentiment API. we used sentiment API without problems for 3 or 4 weeks. Then, we sometimes faced TargetInvocationException after calling sentiment API

System.Reflection.TargetInvocatinException : An开发者_开发知识库 exception occurred during the operation, making the result invalid. Check InnerExcepton for exception details. ---> System.Net.WebException: The remote server returned an error: NotFound ---> System.Net.WebException : The remote server returned an error: NotFound. at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult ayncResult) at System.Net.Browser.BrowserHttpWebRequest.<>c_DisplalyClass4.b__1(Object sendState) ---End of inner exception stack trace at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state) at System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult ayncResult) at System.Net.WebClient.GetWebResponse(WebRequest, IAsyncResult result) at System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result) ---End of inner exception stack trace at System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary() at System.Net.DownloadStringCompletedEvent Args.get_Result() at SilverlightApplication1.MainPage.<>c_DisplayClass1a.<>c_DisplayClass23.b__16(Object ss, DownloadStringCompletedEventArgs en)

I faced this exception when I had a data contract class which had a property of type Enum. Then I decorated the DataContract with [KnownType (typeof(MyEnum))]. I did not see this exception after that.