JPA/Hibernate Static Metamodel Attributes not Populated -- NullPointerException

I would like to use JPA2 Criteria API with metamodel objects, which seems to be pretty easy:

...
Root<JPAAlbum> albm = cq.from(JPAAlbum.class);
... albm.get(JPAAlbum_.theme) ... ;

but this Root.get always throws a NullPointerException. JPAAlbum_.theme was automatically generated by Hibernate and looks like

public static volatile SingularAttribute<JPAAlbum, JPATheme> theme;

but it's obviously never populated.

Am I missing a step in the initialization of the framework ?

EDIT: here is a snippet of how I use JPA and the metamodel when it's crashing:

    CriteriaBuilder cb = em.getCriteriaBuilder();

    CriteriaQuery<JPAAlbum> cq = cb.createQuery(JPAAlbum.class) ;
    Root<JPAAlbum> albm = cq.from(JPAAlbum.class);
    cq.where(cb.equal(albm.get(JPAAlbum_.theme).get(JPATheme_.id),
                        session.getTheme().getId())) ;

(JPAAlbum_ is a class, so I just import before) and the associated stacktrace:

Caused by: java.lang.NullPointerException
    at org.hibernate.ejb.criteria.path.AbstractPathImpl.get(AbstractPathImpl.java:138)
    at net.wazari.dao.jpa.WebAlbumsDAOBean.getRestrictionToAlbumsAllowed(WebAlbumsDAOBean.java:55)

EDIT 2:

In the JBoss EntityManager guide, I can see that

When the Hibernate EntityManagerFactory is being built, it will look for a canonical metamodel class for each of the managed typed is knows about and if it finds any it will inject the appropriate metamodel information into them, as outlined in [JPA 2 Specification, section 6.2.2, pg 200]

I could also verify with

     for (ManagedType o : em.getMetamodel().getManagedTypes()) {
            log.warn("___") ;
            for (Object p : o.getAttributes()) {