为什么根据get_kline_serial获取的历史K线数据开高低收都是一个值

symbol CFFEX.IF1801
duration 1800
Name: 21, dtype: object
datetime 1514874600000000000.0
id 247.0
open 4093.8
high 4093.8
low 4093.8
close 4093.8
volume 0.0
open_oi 23625.0
close_oi 23625.0