NPE when firing a null object

I a bean with type parameters I have

@Inject
@Locked
private Event<Lock> locked;

Lock<U, O> newLock = lockStorage.lock(user, object);
locked.fire(newLock);

but it causes an

java.lang.NullPointerException
at org.jboss.weld.util.Observers.checkEventObjectType(Observers.java:52)
at org.jboss.weld.manager.BeanManagerImpl.fireEvent(BeanManagerImpl.java:563)
at org.jboss.weld.event.EventImpl.fire(EventImpl.java:68)
at fi.affecto.marela.framework.locking.LockManager.lock(LockManager.java:87)