EntityManager上的NPE close（）

Question

我想弄清楚为什么这段代码会抛出NPE：

@RequestMapping(value = "/share/{id}/store/{typeId}", method = RequestMethod.PUT)
public ResponseEntity<Share> updateShareStore(@PathVariable final long id,
                                         @PathVariable final String typeId) {
    EntityManagerFactory emf = EMF.get();
    EntityManager em = emf.createEntityManager();
    Share share = null;
    logger.debug(User.class.getName());
    try {
        share = em.find(Share.class, id);
        Store store = em.find(Store.class, typeId);
        share.getStores().add(store);
        em.merge(share);
    } finally {
        em.close(); // NPE Here!
    } 
    return new ResponseEntity<Share>(share,HttpStatus.OK);
}

错误：

java.lang.NullPointerException 
    at com.google.appengine.datanucleus.StoreFieldManager.processPersistable(StoreFieldManager.java:453)
    at com.google.appengine.datanucleus.StoreFieldManager.getDatastoreObjectForCollection(StoreFieldManager.java:987)
    at com.google.appengine.datanucleus.StoreFieldManager.storeRelations(StoreFieldManager.java:877)
    at com.google.appengine.datanucleus.DatastorePersistenceHandler.updateObject(DatastorePersistenceHandler.java:426)
    at org.datanucleus.state.JDOStateManager.flush(JDOStateManager.java:3832)
    at org.datanucleus.ObjectManagerImpl.flushInternalWithOrdering(ObjectManagerImpl.java:3884)
    at org.datanucleus.ObjectManagerImpl.flushInternal(ObjectManagerImpl.java:3807)
    at org.datanucleus.ObjectManagerImpl.flush(ObjectManagerImpl.java:3747)
    at org.datanucleus.ObjectManagerImpl.preCommit(ObjectManagerImpl.java:4137)
    at org.datanucleus.ObjectManagerImpl.transactionPreCommit(ObjectManagerImpl.java:428)
    at org.datanucleus.TransactionImpl.internalPreCommit(TransactionImpl.java:400)
    at org.datanucleus.TransactionImpl.commit(TransactionImpl.java:288)
    at org.datanucleus.ObjectManagerImpl.close(ObjectManagerImpl.java:1090)
    at org.datanucleus.api.jpa.JPAEntityManager.close(JPAEntityManager.java:193)
    at org.demo.resources.ShareController.updateShare(ShareController.java:83)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)

可能是因为我使用em有两种对象类型吗？但这不太可能。