[storm] error on ResultSet.count( ) after group_by( )

[Eduardo Willians]

>>> result = broker.find(Ctb, Ctb.expiration < base,
                               Ctb.spc == False).order_by(Ctb.expiration)
>>> result.group_by(Ctb.op_id)
<storm.store.ResultSet object at 0x01A5E290>
>>> result.count()
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    result.count()
  File "d:\SisTemporal 9.01\narnia\geosql\storm\store.py", line 1216, in count
    return int(self._aggregate(Count(expr, distinct)))
  File "d:\SisTemporal 9.01\narnia\geosql\storm\store.py", line 1189,
in _aggregate
    raise FeatureError("Single aggregates aren't supported after a "
FeatureError: Single aggregates aren't supported after a  GROUP BY clause
>>>

Why this?