Свойство null-null ссылается на значение null или переходное значение

Опасность проблемы с сохранением родительского/дочернего объекта в спящем режиме. Любая идея будет высоко оценена.

org.hibernate.PropertyValueException: not-null property references a null or transient value: example.forms.InvoiceItem.invoice
    at org.hibernate.engine.Nullability.checkNullability(Nullability.java:100)
        .... (truncated)

hibernate mapping:

<hibernate-mapping package="example.forms">
    <class name="Invoice" table="Invoices">
        <id name="id" type="long">
            <generator class="native" />
        </id>
        <property name="invDate" type="timestamp" />
        <property name="customerId" type="int" />
        <set cascade="all" inverse="true" lazy="true" name="items" order-by="id">
            <key column="invoiceId" />
            <one-to-many class="InvoiceItem" />
        </set>
    </class>
    <class name="InvoiceItem" table="InvoiceItems">
        <id column="id" name="itemId" type="long">
            <generator class="native" />
        </id>
        <property name="productId" type="long" />
        <property name="packname" type="string" />
        <property name="quantity" type="int" />
        <property name="price" type="double" />
        <many-to-one class="example.forms.Invoice" column="invoiceId" name="invoice" not-null="true" />
    </class>
</hibernate-mapping>

InvoiceManager.java

class InvoiceManager {

    public Long save(Invoice theInvoice) throws RemoteException {
        Session session = HbmUtils.getSessionFactory().getCurrentSession();
        Transaction tx = null;
        Long id = null;
        try {
            tx = session.beginTransaction();
            session.persist(theInvoice);
            tx.commit();
            id = theInvoice.getId();
        } catch (RuntimeException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
            throw new RemoteException("Invoice could not be saved");
        } finally {
            if (session.isOpen())
                session.close();
        }
        return id;
    }
}

Invoice.java

public class Invoice implements java.io.Serializable {
    private Long id;
    private Date invDate;
    private int customerId;
    private Set<InvoiceItem> items;

    public Long getId() {
        return id;
    }
    public Date getInvDate() {
        return invDate;
    }
    public int getCustomerId() {
        return customerId;
    }
    public Set<InvoiceItem> getItems() {
        return items;
    }
    void setId(Long id) {
        this.id = id;
    }
    void setInvDate(Date invDate) {
        this.invDate = invDate;
    }
    void setCustomerId(int customerId) {
        this.customerId = customerId;
    }
    void setItems(Set<InvoiceItem> items) {
        this.items = items;
    }
}

InvoiceItem.java

public class InvoiceItem implements java.io.Serializable {
    private Long itemId;
    private long productId;
    private String packname;
    private int quantity;
    private double price;
    private Invoice invoice;

    public Long getItemId() {
        return itemId;
    }
    public long getProductId() {
        return productId;
    }
    public String getPackname() {
        return packname;
    }
    public int getQuantity() {
        return quantity;
    }
    public double getPrice() {
        return price;
    }
    public Invoice getInvoice() {
        return invoice;
    }
    void setItemId(Long itemId) {
        this.itemId = itemId;
    }
    void setProductId(long productId) {
        this.productId = productId;
    }
    void setPackname(String packname) {
        this.packname = packname;
    }
    void setQuantity(int quantity) {
        this.quantity = quantity;
    }
    void setPrice(double price) {
        this.price = price;
    }
    void setInvoice(Invoice invoice) {
        this.invoice = invoice;
    }
}