hibernate: attempt to lock a deleted instance

Question

I'm trying to lock an entity while working on it, and I'm doing the following:
1: retrieveing the entity from the database using this method:

@Override
public Optional<T> findById(final Serializable id) {
    Session session = getSession();
    T e = session.get(persistentClass, id);
    return Optional.ofNullable(e);
}

2: locking the entity:

public T myMethod(final Serialzable id){
   //...
   T entity = getById(id);
   LockRequest lr = session.buildLockRequest(LockOptions.UPGRADE);
   lr.setTimeOut(LOCK_REQUEST_TIME_OUT);
   lr.lock(entity); // <-- problem is in this line
   //...
}

but I get this exception:

org.hibernate.ObjectDeletedException: attempted to lock a deleted instance: [com.x.y.Entity#1]

As you can see I'm just retrieving the entity and locking it, not sure why the entity is in the deleted state!!