[Python] sqlalchemy.exc.InvalidRequestError: SQL expression, column, or mapped entity expected

I am running into this sqlachemy error that I haven't been able to understand:

sqlalchemy.exc.InvalidRequestError: SQL expression, column, or mapped entity expected - got '<class '__main__.JobRecord'>'


What does this error mean? What are possible causes?

This is the method that triggers the error:

@classmethod
def find_job_record_from_pk(cls, pk):
'''
return the job record with the given pk
'''
job_record = MlcDb.get_session().query(cls).filter(cls.pk == pk).first()
return job_record


Mapping:

@classmethod
def define_mapping(cls):
'''
SQLAlchemy mapping definition
'''
cls.mapper = mapper(cls, cls.table,
polymorphic_on = cls.table.c.item_type,
properties = {
'item_type': synonym('_JobRecord__item_type', map_column=True),
'version': synonym('_JobRecord__version', map_column=True),
'state': synonym('_JobRecord__state', map_column=True),
'date_created' : synonym( '_JobRecord__date_created', map_column=True)
}
)

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