Merge throw exception "Attempt to reuse an existing primary key value"

import javax.persistence.*;

public class ZeroIdTest {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("objectdb:$objectdb/db/test.tmp;drop");

        EntityManager em = emf.createEntityManager();
        final int Id = 0;
        em.getTransaction().begin();
        em.persist(new MyEntity(Id));
        em.getTransaction().commit();
        em.getTransaction().begin();
        em.merge(new MyEntity(Id));
        em.getTransaction().commit();
        em.close();
        emf.close();
    }

    @Entity
    public static class MyEntity {
        @Id
        private int Id;

        public MyEntity(int id) {
            Id = id;
        }

        public MyEntity() {
        }
    }