symbols=['IF888.CFFEX']
freq='1d'
begintime=datetime.datetime.strptime('20181001','%Y%m%d')
endtime=datetime.datetime.strptime('20190101','%Y%m%d')
bars=tbpy.get_history(symbols, freq,begintime, endtime, fields=None, timeout='30s')
code=symbols[0]
ddrb=bars[code]
以上执行显示KeyError: 'IF888.CFFEX'是什么原因？