failure to enforce NOT NULL for java.lang.String

@Entity
public class Request
{
  public Request() { }

  @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
  private Long id;

  @Unique
  @Persistent(nullValue=NullValue.EXCEPTION)
  private String username;

  public Long getId() { return id; }
  public void setId(Long id) { this.id = id; }

  public String getUsername() { return username; }
  public void setUsername(String username) { this.username = username; }
}

public class RegressionTest
{
  /**
   */
  @Test
  public void regressionTest()
  {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("target/test.odb;drop");
    EntityManager em = emf.createEntityManager();
    EntityTransaction tr = em.getTransaction();

    Request r = new Request();
    r.setUsername(null);

    //try {
      tr.begin();
      em.persist(r);
      tr.commit();
    //}
    //catch (...) { }

    em.close();
    emf.close();
  }
}

import javax.jdo.annotations.*;
import javax.persistence.*;


public class T744 {

    public static void main(String [] args){
        EntityManagerFactory emf =
            Persistence.createEntityManagerFactory(
                "objectdb:test.tmp;drop");
        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();
        em.persist(new Request());
        em.getTransaction().commit();

        em.close();
        emf.close();
    }

    @Entity
    public static class Request {

        public Request() {
        }

        @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
        private Long id;

        @Unique
        //@Persistent(nullValue=NullValue.EXCEPTION)
        @Basic(optional=false)
        private String username;
       
        //@Basic(optional=false)
        //@Persistent(nullValue=NullValue.EXCEPTION)
        //private Request otherRequest;

        public Long getId() { return id; }
        public void setId(Long id) { this.id = id; }

        public String getUsername() { return username; }
        public void setUsername(String username) { this.username = username; }
    }
}