I have following error while trading. I have defined fixed price strategy and session:

[```
2018-01-04 03:10:18,219 I] cointrol.trader.workers.Trader: will try again
[2018-01-04 03:10:21,225 E] cointrol.trader.workers.Trader: work failed
Traceback (most recent call last):
File "/home/rovshan/development/Cointrol/cointrol/cointrol/trader/workers.py", line 68, in run_forever
result = yield self.work()
File "/home/rovshan/development/Cointrol/virtualenv/lib/python3.6/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
File "/home/rovshan/development/Cointrol/virtualenv/lib/python3.6/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
File "", line 4, in raise_exc_info
File "/home/rovshan/development/Cointrol/virtualenv/lib/python3.6/site-packages/tornado/gen.py", line 307, in wrapper
yielded = next(result)
File "/home/rovshan/development/Cointrol/cointrol/cointrol/trader/workers.py", line 181, in work
trading_session = account.get_active_trading_session()
File "/home/rovshan/development/Cointrol/cointrol/cointrol/core/models.py", line 79, in get_active_trading_session
session.set_status(FINISHED)
File "/home/rovshan/development/Cointrol/cointrol/cointrol/core/models.py", line 186, in set_status
assert self.became_finished is None
AssertionError