GAE JDO NullPointerException при операции .add ()

Я пытаюсь сделать отношения "один ко многим" в GAE, и когда я добавляю в список, я получаю следующую ошибку:

[java] java.lang.NullPointerException
 [java]     at com.google.appengine.datanucleus.scostore.FKListStore.getIndexPropertyName(FKListStore.java:965)
 [java]     at com.google.appengine.datanucleus.scostore.FKListStore.getFilterPredicates(FKListStore.java:940)
 [java]     at com.google.appengine.datanucleus.scostore.FKListStore.listIterator(FKListStore.java:383)
 [java]     at com.google.appengine.datanucleus.scostore.FKListStore.listIterator(FKListStore.java:349)
 [java]     at com.google.appengine.datanucleus.scostore.FKListStore.iterator(FKListStore.java:342)
 [java]     at org.datanucleus.store.types.sco.backed.List.loadFromStore(List.java:304)
 [java]     at org.datanucleus.store.types.sco.backed.List.add(List.java:685)
 [java]     at Utils.GardenUtils.PostMessage(GardenUtils.java:187)
 [java]     at Servlets.GardenServlets.PostGardenMessageServlet.processRequest(PostGardenMessageServlet.java:28)
 [java]     at Servlets.GardenServlets.PostGardenMessageServlet.doPost(PostGardenMessageServlet.java:70)
 [java]     at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
 [java]     at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
 [java]     at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
 [java]     at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:127)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectRequest(DevAppServerModulesFilter.java:366)
 [java]     at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectModuleRequest(DevAppServerModulesFilter.java:349)
 [java]     at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:116)
 [java]     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
 [java]     at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
 [java]     at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
 [java]     at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
 [java]     at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
 [java]     at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
 [java]     at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98)
 [java]     at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
 [java]     at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:491)
 [java]     at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
 [java]     at org.mortbay.jetty.Server.handle(Server.java:326)
 [java]     at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
 [java]     at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
 [java]     at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
 [java]     at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
 [java]     at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
 [java]     at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
 [java]     at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

Вот как я добавляю нового члена списка:

    public static void PostMessage(String userID, String gardenKey, String content) {
    PersistenceManager pm = PMF.get().getPersistenceManager();

    try {
        User user = pm.getObjectById(User.class, userID);

        GardenMessage message = new GardenMessage();
        message.setFirstName(user.getFirstName());
        message.setLastName(user.getLastName());
        message.setUserID(userID);
        message.setContent(content);
        message.setGardenKey(gardenKey);
        message.setPostDate(new Date());

        Garden garden = pm.getObjectById(Garden.class, gardenKey);
        garden.getMessages().add(message);
    } finally {
        pm.close();
    }
}

В классе Garden у меня есть следующее:

@Persistent
private List<GardenMessage> messages;

Пожалуйста, дайте мне знать, что не так с моим кодом.