SQLAlchemy OrderingList misbehavior?

test_ordering_list.py

from sqlalchemy import Column, Integer, ForeignKey, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.orderinglist import ordering_list
from sqlalchemy.orm import relationship, backref, sessionmaker


Base = declarative_base()


class Slide(Base):
    __tablename__ = 'slide'
    id = Column(Integer, primary_key=True)


class Bullet(Base):
    __tablename__ = 'bullet'
    id = Column(Integer, primary_key=True)
    slide_id = Column(Integer, ForeignKey('slide.id'))
    position = Column(Integer)
    slide = relationship(
        "Slide",
        backref=backref(
            "bullets",
            order_by="Bullet.position",
            collection_class=ordering_list('position')))


engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)

session = Session()
s = Slide()
session.add(s)
session.commit()
session.close()

session = Session()
b = Bullet()
b.slide = s
session.add(b)
session.commit()
session.refresh(b)
session.close()

assert b.position is None  # But I expect it to not be None

session = Session()
s = Slide()
session.add(s)
session.commit()
session.refresh(s)
s_id = s.id
session.close()
del s

session = Session()
b = Bullet()
b.slide_id = s_id
session.add(b)
session.commit()
session.refresh(b)
session.close()

assert b.position is None  # But I expect it to not be None