JPA Hibernate illegal access to loading collection

I have used hibernate annotation for a while, and am just now running into a problem when trying a slightly complicated mapping.

The class hierarchy is like this

// no @Entity annotation here
public abstract class Entity implements CommonEntity {
...
}

@javax.persistence.Entity
@Table(name="FLOW_ELEMENT")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
@DiscriminatorColumn(
    name="elementType",
    discriminatorType=DiscriminatorType.STRING
)
public abstract class FlowDiagramObject extends Entity implements Serializable {
      @Id
      @Column(name = "FLOW_ELEMENT_ID", precision = 22, scale = 0)
      @GeneratedValue(strategy = GenerationType.AUTO)
      protected long id;

.......
}
@javax.persistence.Entity
@DiscriminatorValue("Diagram")
public class FlowDiagram extends FlowDiagramObject implements Serializable {

    @OneToMany(mappedBy="parentFlow", cascade={CascadeType.ALL})
      @Cache(usage= CacheConcurrencyStrategy.READ_WRITE)
      protected Set<FlowElement> elements = new HashSet<FlowElement>();      
.....

}
@Entity
public abstract class FlowElement extends FlowDiagramObject {
      @ManyToOne
      @JoinColumn (name="parent_object_id")
      protected FlowDiagram parentFlow;            // only used to point to diagram element
      
......................
}

@javax.persistence.Entity
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
@DiscriminatorValue("CommandElement")
public class CommandElement extends FlowElement {

....
}

I am able to using HQL to get a FlowDiagram object, but when accessing its children

flowDiagram.getElements();

I get a illegal access to loading collection exception.

I am completely sure this is done inside the session, so that is not the problem. There must be something wrong with my mapping.

Anyone see the problem?