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problem with java.util.Map

I've run into some problems using TreeMaps with ozone 0.5.3, Java 1.3, 
Win98.

Trying to return any entrySet (or keySet) results in a 
java.lang.NullPointerException. Is this a programming error? See example 
below.

Thanks very much,

don

-------------------- Local.java

import java.util.*;
import org.ozoneDB.*;


public class Local extends Object {

    public static void main ()throws Exception {

        LocalDatabase db = new LocalDatabase();
        try {
            db.open ("/tmp/db");
            }
        catch (Exception e) {
            System.out.println ("No DB found, creating...");
            db.create ("/tmp/db");
            db.open ("/tmp/db");
            }

        db.reloadClasses();
        System.out.println ("connected...");

        MapTester mapTester = (MapTester)db.createObject 
(MapTesterImpl.class.getName(), OzoneInterface.Public, "maptester");
        mapTester.put("key1", "value1");
	  mapTester.put("key2", "value2");
	  Set entries = mapTester.entrySet();
	  db.close();
        System.out.println ("deconnected...");
        }

    }

------------------------- MapTester.java

import org.ozoneDB.*;
import java.util.*;


public interface MapTester extends OzoneRemote  {

   public void put (Object key, Object value) ; /*update*/

   public Set entrySet();

   }

------------------------ MapTesterImpl.java

import org.ozoneDB.*;
import java.util.*;


public class MapTesterImpl extends OzoneObject implements MapTester {


   public MapTesterImpl() {
       map = new TreeMap();
   }

   public void put (Object key, Object value) {
      map.put(key, value);
   }

   public Set entrySet() {
      // the exception occurs here
      return map.entrySet();
   }

   private Map map;
   }
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