Mocking EntityManager

I am getting NPE while mocking EntityManager, below is my code,

@Stateless
public class NodeChangeDeltaQueryBean implements NodeChangeDeltaQueryLocal {

    @PersistenceContext
    private EntityManager em;
    @Override
    public String findIdByNaturalKey(final String replicationDomain, final int sourceNodeIndex,
                                     final int nodeChangeNumber) {
        List<String> result =
            NodeChangeDelta.findIdByNaturalKey(this.em, replicationDomain, sourceNodeIndex,
                nodeChangeNumber).getResultList();
        return result.isEmpty() ? null : result.get(0);
    }
}

My Entity Class

@Entity
public class NodeChangeDelta implements Serializable, Cloneable, GeneratedEntity, KeyedEntity<String> {

public static TypedQuery<String> findIdByNaturalKey(final EntityManager em, final String replicationDomain, final int sourceNodeIndex, final int nodeChangeNumber) {
        return em.createNamedQuery("NodeChangeDelta.findIdByNaturalKey", String.class)
            .setParameter("replicationDomain", replicationDomain)
            .setParameter("sourceNodeIndex", sourceNodeIndex)
            .setParameter("nodeChangeNumber", nodeChangeNumber);
    }
}

My Test Class

@RunWith(MockitoJUnitRunner.class)
public class NodeChangeDeltaQueryBeanTest {

    @InjectMocks
    NodeChangeDeltaQueryBean nodeChangeDeltaQueryBean;

    @Mock
    EntityManager em;

@Test
    public void testFindIdByNaturalKey() {
        this.addNodeChangeDelta();
        this.nodeChangeDeltaQueryBean.findIdByNaturalKey(this.REPLICATION_DOMAIN,
            this.SOURCE_NODE_INDEX, this.NODE_CHANGE_NUMDER);
    }
}

While debugging em is not null (also other arguments REPLICATION_DOMAIN, SOURCE_NODE_INDEX, NODE_CHANGE_NUMDER not null) in Entity class, whereas em.createNamedQuery("NodeChangeDelta.findIdByNaturalKey", String.class) is null.

please help to find and solve the issue?