Simplest Invoice add failing - UNEXPECTED_ERROR

The following code consistently returns 'unexpected error', if I modify the customer to something incorrect then I do get a bad lookup on it.  Does anyone have any advice on how to proceed?  I'm trying to work up from a very simple add but I get an obscure error even at that.

Thanks in advance,
Derek.

        public void processInvoices()
        {
            Passport thisPassport = new Passport();

            thisPassport.account = "999999";
            thisPassport.email = "dumbfounded@islandcore.com";              
            thisPassport.password = "password";
            RecordRef XMLImporter = new RecordRef();
            XMLImporter.name = "Administrator";
            thisPassport.role = XMLImporter;

            nsService.CookieContainer = new CookieContainer();

            Status status = nsService.login(thisPassport).status;
            Invoice inv = getSimpleInvoice();
         
            WriteResponse response = nsService.add(inv);