NPE in HibernateContext.prepareSession

Hi,
this might be a pilot error ...

When I try to obtain a session like this:

Session =
HibernateContext.getSession("java:/hibernate/SessionFactory");

I get a NPE

Caused by: java.lang.NullPointerException
at
org.jboss.hibernate.session.HibernateContext.prepareSession(HibernateContext.java:171)
at
org.jboss.hibernate.session.HibernateContext.getSession(HibernateContext.java:99)
at
de.bsd.adb_hibernate.server.ServerFacade.beispiel(ServerFacade.java:22)

If I do it the classical way

InitialContext ic = new InitialContext();
Object o = ic.lookup("java:/hibernate/SessionFactory");
sf = (SessionFactory)o;

Session sess = sf.openSession()

it works as intended.