Silverlight: Exception in AsyncCallback

The code blow works, it creates a task, but the AsyncCallback in BeginExcecute throws an exception that I can't catch?

(the method appendDescription adds given information to a textfield in CRM)

private void createTask(string summary) 
        {
            OrganizationRequest request = new OrganizationRequest() { RequestName = "Create" };
            Entity entity = new Entity();
            entity.LogicalName = "task";
            entity.Attributes = new AttributeCollection();

            entity.Attributes.Add(new SoapForSilverlightSample.CrmSdk.KeyValuePair<string, object>()
                {
                    Key = "subject",
                    Value = txtSummery.Text.Trim()
                });

            entity.Attributes.Add(new SoapForSilverlightSample.CrmSdk.KeyValuePair<string, object>()
            {
                Key = "regardingobjectid",
                Value = new EntityReference() { LogicalName = "incident", Id = getIncidentID() }
            });

            //request properties need to be explicitly named as strings 
            request["Target"] = entity;

            appendDescription("Start", "[createTask]");

            try
            {
                IOrganizationService service = SoapForSilverlightSample.SilverlightUtility.GetSoapService();
                //depending on how you do things your calls will most likely be asynchronous 
                service.BeginExecute(request, new AsyncCallback(CrmCreate_Callback), service);

                appendDescription("Saved!", "[createTask]");
            }
            catch(Exception ex)
            {
                appendDescription(ex.Message, "[Excption => createTask]");
            }
        }

        private void CrmCreate_Callback(IAsyncResult result)
        {
            
            OrganizationResponse response = ((IOrganizationService)result.AsyncState).EndExecute(result);
            Guid results = (Guid)response["id"];
            Guid CreatedEntity = results;

            appendDescription("utan EndExecute", "[CrmCreate_Callback]");
        }

The exception

Unhandled Error in Silverlight Application An AsyncCallback threw an exception.   at System.ServiceModel.AsyncResult.Complete(Boolean completedSynchronously)
   at System.ServiceModel.Channels.HttpOutput.WebRequestHttpOutput.GetOutputStreamAsyncResult.OnGetRequestStream(IAsyncResult result)
   at System.Net.Browser.BrowserHttpWebRequest.<>c__DisplayClass14.<InvokeGetRequestStreamCallback>b__12(Object state2)
   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()