回测时，同一根分钟k线成交量第一次变化时总是0，要第二次才返回数值

api = TqApi(backtest=TqBacktest(start_dt=date(2020,2,27), end_dt=date(2020,2,28)))
klines = api.get_kline_serial("KQ.m@SHFE.rb", 60)
 while True:
    api.wait_update()
    if api.is_changing(klines):
        print('{} / - {}'.format(time_to_str(klines.iloc[-1]['datetime']), 
                                    klines.iloc[-1]['volume']))