OneToMany bi-directional save throws java.lang.IncompatibleClassChangeError: Class org.hibernate.collection.internal.PersistentMap does not implement the requested interface java.util.Collection

Hibernate ORM
1

I am using hibernate version 5.4.6 final with JPA and Java 8.
I have a One to many bi-directional relationship between entities.
Entity uses a Map to represent items, they are as below. Full entity classes are here.

@Entity
@Access(AccessType.FIELD)
public class E_Cart {
    @Id
    @GeneratedValue(generator = "cartSeq",
            strategy = GenerationType.SEQUENCE)
    private long id;

    @OneToMany(mappedBy = "cart")
    @MapKey(name = "version")
    private Map<String, E_CartItem> cartItemMap = new HashMap<>();
    // skipping getter  and setters
}

@Entity
@Access(AccessType.FIELD)
public class E_CartItem {
    @Id
    @GeneratedValue(generator = "cartItemSeq",
            strategy = GenerationType.SEQUENCE)
    private long id;

    @ManyToOne
    private E_Cart cart;

    private String version;
}

If I am trying to save E_CartItem,

entityManager.getTransaction().begin();
E_Cart cart = new E_Cart();
entityManager.persist(cart);

E_CartItem ci = new E_CartItem();
ci.setVersion("one");
ci.setCart(cart);
entityManager.persist(ci);

entityManager.getTransaction().commit();

I am getting below exception:

java.lang.IncompatibleClassChangeError: Class org.hibernate.collection.internal.PersistentMap does not implement the requested interface java.util.Collection
at com.org.entity.E_CartItem.$$_hibernate_write_cart(E_CartItem.java)
	at com.org.entity.E_CartItem.setCart(E_CartItem.java:46)
	at org.hibernate.bugs.JPAUnitTestCase.hhh1Test(JPAUnitTestCase.java:45)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)

Questions:

  1. Am I doing it in correct way?
  2. I can see the exception is pointing to method $$_hibernate_write_cart in the class file generated by the enhancer.

I have created a branch here, with sample test case.

Requesting any one to suggest what changes are needed so that entity could be saved?