Stateful (statless) EJB3 and inheritance

[trofimov]

I create:

public abstract class GenericEjb3DAO <T, ID extends Serializable> implements GenericDAO<T, ID> {

 @PersistenceContext
 public EntityManager em;
...
}

Then i create:

@Stateless
public class UserDAOBean extends GenericEjb3DAO<User, Long> implements UserDAO {
..

..
}

And when i handle to "em" in first or second class, exception is raised
"java.lang.NullPointerException".

When i remove inheritance, all fine. What is wrong in my code?
Thanx.