Problem with Binary column as a primary key and the session cache

Running the following code gives the following error. I tried to make it as small as possible.

import sqlalchemy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine, Column, Binary

Base = declarative_base()

class Blob(Base):
    __tablename__ = "blob"
    b = Column(Binary, primary_key=True)
    def __init__(self):
        self.b = "abcd"

if __name__ == "__main__":
    engine = create_engine("sqlite://")
    Base.metadata.create_all(engine)
    Session = sessionmaker(bind=engine)
    s = Session()
    s.add(Blob())
    s.commit()
    print repr(s.query(Blob).first().b)

c:\backend>test.py
Traceback (most recent call last):
  File "c:\backend\test.py", line 21, in <module>
    print repr(s.query(Blob).first().b)
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\query.py", line 1231, in first
    ret = list(self[0:1](0:1))
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\query.py", line 1152, in __getitem__
    return list(res)
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\query.py", line 1353, in instances
    rows = [process[0](process[0)(context, row) for row in fetch]
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\query.py", line 1955, in main
    return _instance(row, None)
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\mapper.py", line 1614, in _instance
    if identitykey in session_identity_map:
  File "C:\software\python25\Lib\site-packages\sqlalchemy\orm\identity.py", line 95, in __contains__
    if dict.__contains__(self, key):
TypeError: writable buffers are not hashable

When I change the column definition from

    b = Column(Binary, primary_key=True)

to

    id = Column(Integer, primary_key=True)
    b = Column(Binary)

the error goes away. It seems that the Binary column is converted to a buffer object and the orm tries to use it as a key for a regular dict.

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