EclipseLink merge unexpected cascade

I have two entity classes user and device.

User entity:

public class User {
    private Long userId;

    @OneToMany( mappedBy = "userId", fetch = FetchType.LAZY)
    private Collection<Device> deviceCollection;

and device entity:

public class Device implements Serializable {
    @JoinColumn(name = "user_id", referencedColumnName = "user_id")
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    private User userId;

When I merge a previously detached device entity into the entity manager after the parent user has been deleted, both the (previously removed) user and the device are re-inserted into the database. There is no cascade annotation on user or device entity; therefore, I don't expect the user entity to be reinserted but it did;

How do I prevent the merge operation to cascade to the user entity?

Thanks in advance.