google-app-engine - polymodel は KeyProperty でうまく動作しないようです

Question

KeyProperty のkind検証が のサブクラスに対して機能しないようですPolyModel。

from google.appengine.ext import ndb
from google.appengine.ext.ndb import polymodel


class Item(polymodel.PolyModel):
    parent = ndb.KeyProperty(kind="Folder")

class Folder(Item):
    title = ndb.StringProperty()

    def add_item(self, item):
        item.set_parent(self.key)

class File(Item):
    pass


class Main(webapp2.RequestHandler):
    def get(self):
        rootfolder = Folder(title="root")
        rootfolder.put()

        # the next line raise exception
        subfolder = Folder(title="Cool things", parent=rootfolder.key) 
        subfolder.put()

例外：

line 1935, in _validate
    'Expected Key with kind=%r, got %r' % (self._kind, value))
BadValueError: Expected Key with kind='Folder', got Key('Item', 6544293208522752)

Can ndb.KeyProperty reference a base model class when using model inheritance?で Guido van Rossum が言ったのと同じように見え ますか?