SQLAlchemy driver fails in get_meter_statistics when the EventFilter has the 'end' parameter set

Sample test code:

    def test_by_user_period2(self):
        f = storage.EventFilter(
            user='user-5',
            meter='volume.size',
            start='2012-09-25T10:28:00',
            end='2012-09-25T11:28:00',
            )
        results = self.conn.get_meter_statistics(f, period=1800)
        self.assertEqual(len(results), 2)

Traceback (most recent call last):
  File "/home/stas/Cloud/ceilometer/tests/storage/base.py", line 775, in test_by_user_period2
    results = self.conn.get_meter_statistics(f, period=1800)
  File "/home/stas/Cloud/ceilometer/ceilometer/storage/impl_sqlalchemy.py", line 451, in get_meter_statistics
    + datetime.timedelta(seconds=i * period))
TypeError: unsupported operand type(s) for +: 'NoneType' and 'datetime.timedelta'