MarkErrorAsHandled() doesn't work on InvokeServerMethodOperation Coroutine

   Running DevForce 6.0.9 under Silverlight 4. I have a server side invoke method that looks like this:

public InvokeServerMethodOperation GetPasTimeSheetOperation(DateTime? dateTime)
{
    const string typeName = PASManagerServerInvokeConsts.TypeName;
    const string methodName = PASManagerServerInvokeConsts.MethodNames.GetTimeSheet;

    var pasImportRequest = new PASImportRequest { UserName = PASUserName, Password = PASPassword, WeekEnding = dateTime };

    return Manager.InvokeServerMethodAsync(
        typeName,
        methodName,
        null,
        null,
        pasImportRequest
        );
}

Which gets called in a co-routine like this:

var getPasTimeSheetOperation = _timeSheetRepository.GetPasTimeSheetOperation(_selectedDate);

yield return getPasTimeSheetOperation;            

if (getPasTimeSheetOperation.HasError)
{
    getPasTimeSheetOperation.MarkErrorAsHandled();
}

However if the server side method throws an exception it flow to the main application. The only way to prevent this is call the MarkErrorAsHandled() in the InvokeServerMethodAsync callback like this:

public InvokeServerMethodOperation GetPasTimeSheetOperation(DateTime? dateTime)
{
    const string typeName = PASManagerServerInvokeConsts.TypeName;
    const string methodName = PASManagerServerInvokeConsts.MethodNames.GetTimeSheet;

    var pasImportRequest = new PASImportRequest { UserName = PASUserName, Password = PASPassword, WeekEnding = dateTime };

    return Manager.InvokeServerMethodAsync(
        typeName,
        methodName,
        HandlerToEatErrors,
        null,
        pasImportRequest
        );
}

private void HandlerToEatErrors(InvokeServerMethodOperation op)
{
    op.MarkErrorAsHandled();           
}

This seems a little kludgy, is this a defect or am I doing something wrong?