Calling NAV Web Service from Java

My NAV webservices use XMLPort references. I got this from: http://kauffmann.nl/index.php/2011/06/0 ... rvices-3/.. Calling the services in .Net is works perfectly. However using the same concept in Java gives problems. Here is my java call:

public static void main(String[] args) {

Webservice service = new USSDWebservice();
WebservicePort port = service.getUSSDWebservicePort();

Holder<NavCustomers> NavCustomer = new Holder<NavCustomers>();
Holder<Programmes> Programme = new Holder<Programmes>();

port.getStudent(NavCustomer, "P0128549D");

System.out.println("\n Dynamics NAV Result: " + NavCustomer.value);
System.out.println("\nTHE END");
}

I get an error from the NAV server:

Parameter XXX in method XXXX in service Webservice is null! What am i missig?