Mutable + column properties broken

Trying to use both mutable and column properties within the same table results in mapper exception.

The stacktrace is:

Traceback (most recent call last):
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 682, in __getattr__
    return getattr(self.comparator, key)
AttributeError: 'Comparator' object has no attribute 'info'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/srv/www/tmp.sakuya.pl/f/test", line 25, in <module>
    session.query(User).all()
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1328, in query
    return self._query_cls(entities, self, **kwargs)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 139, in __init__
    self._set_entities(entities)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 150, in _set_entities
    self._set_entity_selectables(self._entities)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 180, in _set_entity_selectables
    ent.setup_entity(*d[entity])
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 3595, in setup_entity
    self._with_polymorphic = ext_info.with_polymorphic_mappers
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 764, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 1948, in _with_polymorphic_mappers
    configure_mappers()
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 2869, in configure_mappers
    mapper, mapper.class_)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/event/attr.py", line 218, in __call__
    fn(*args, **kw)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/events.py", line 616, in wrap
    fn(*arg, **kw)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/ext/mutable.py", line 603, in listen_for_type
    prop.columns[0].info.get('_ext_mutable_orig_type')
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 688, in __getattr__
    key)
AttributeError: Neither 'Label' object nor 'Comparator' object has an attribute 'info'

The code works fine on 1.1.7 and doesn't on 1.1.8. The GitHub hash of the change that introduces this behavior is 07b63894cb8ff9529b406f196b5d7cb9af209e9e: "Track SchemaEventTarget types in as_mutable()".

I attach a code sample that triggers this behavior.

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