orm query.update() raises KeyError exception after a query.get() has been called

Calling update() causes an exception any time that a record is affected that was previously fetched using get() in the same session.

Observe this example code from the documentation. The last line was added to illustrate how an exception might occur:

from sqlalchemy import *

engine = create_engine('sqlite:///:memory:', echo=True)
metadata = MetaData()

users_table = Table('users', metadata,
    Column('id', Integer, primary_key=True),
    Column('name', String),
    Column('fullname', String),
    Column('password', String)
)

metadata.create_all(engine)

class User(object):
    def __init__(self, name, fullname, password):
        self.name = name
        self.fullname = fullname
        self.password = password
    def __repr__(self):
        return "<User('%s','%s', '%s')>" % (self.name, self.fullname, self.password)

from sqlalchemy.orm import mapper
mapper(User, users_table)

from sqlalchemy.orm import sessionmaker

Session = sessionmaker(bind=engine)
session = Session()

ed_user = User('ed', 'Ed Jones', 'edspassword')
session.add(ed_user)

our_user = session.query(User).filter_by(name='ed').first()

#Blows up...
session.query(User).filter(User.name == 'ed').update({ User.fullname : "new fullname" })

I hope this is clear. This is the stacktrace that is produced:

  File "./sa/test/lib/python2.6/site-packages/SQLAlchemy-0.5.4p2-py2.6.egg/sqlalchemy/orm/query.py", line 1704, in update
    session.expire(session.identity_map[identity_key](identity_key), values.keys())
  File "./sa/test/lib/python2.6/site-packages/SQLAlchemy-0.5.4p2-py2.6.egg/sqlalchemy/orm/session.py", line 959, in expire
    _expire_state(state, attribute_names=attribute_names, instance_dict=self.identity_map)
  File "./sa/test/lib/python2.6/site-packages/SQLAlchemy-0.5.4p2-py2.6.egg/sqlalchemy/orm/state.py", line 217, in expire_attributes
    impl = self.manager[key](key).impl
KeyError: <sqlalchemy.orm.attributes.InstrumentedAttribute object at 0x27fb9d0>

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