KeyError when attempting to merge() polymorphic object when polymorphic_on isn't a Column

sqla 0.7.6 - 0.7.7 (actually 8172:565b5dc537e8c155fb85878d477180a4c954b81f)

Traceback (most recent call last):
  File "merge_polymorphic_on.py", line 62, in <module>
    session.merge(m)
  File "/home/rarch/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.7.601kbdev-py2.6-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1360, in merge
    load=load, _recursive=_recursive)
  File "/home/rarch/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.7.601kbdev-py2.6-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1452, in _merge
    load, _recursive)
  File "/home/rarch/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.7.601kbdev-py2.6-linux-x86_64.egg/sqlalchemy/orm/properties.py", line 149, in merge
    dest_state.expire_attributes(dest_dict, [self.key](self.key))
  File "/home/rarch/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.7.601kbdev-py2.6-linux-x86_64.egg/sqlalchemy/orm/state.py", line 259, in expire_attributes
    impl = self.manager[key](key).impl
KeyError: '_sa_polymorphic_on'

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