Sqlalchemy association_proxy error

sqla.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import *
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.ext.associationproxy import association_proxy
Base = declarative_base()

class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
   

class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    id_a = Column(Integer, ForeignKey('a.id'))
    a = relationship('A', backref='bs')

class C(Base):
    __tablename__ = 'c'
    id = Column(Integer, primary_key=True)
    id_b = Column(Integer, ForeignKey('b.id'))
    b = relationship('B', backref='cs')
    a = association_proxy('b', 'a')

engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)

sess = sessionmaker(engine)()

a1 = A()
b1 = B()
c1 = C()
c2 = C()

a1.bs.append(b1)
b1.cs.append(c1)
sess.add_all([
    a1, b1, c1, c2
])

# querying works
print sess.query(C).filter(C.a==a1)
# access works
print c1.a
# AttributeError: 'NoneType' object has no attribute 'a'
print c2.a