JDO stability in a Google App Engine application with a HashMap child field

Question

JDO stability in a Google App Engine application with a HashMap child field

I have a parent class and I want to save a HashMap in it. However, every time I try to change this HashMap, I get the following error:

PM org.datanucleus.store.appengine.MetaDataValidator checkForIllegalChildField WARNING. It is not possible to verify the one-to-many relationship. Com.monsters.server.MonUser.monsters

Any idea what this is about? Here is the code:

This is the parent class code

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable="true") 
public class MonUser {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent(serialized="true", mappedBy = "owner")
    @Element(dependent = "true")
    private HashMap<String,Monster> monsters;

...

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable="true")
public class Monster {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private MonUser owner;