求解，pandas中使用层次化索引后切片为什么报错了

import pandas as pd
import numpy as np
windex=['a','b','c']
windexd=[1 for i in range(2)]+[0 for i in range(3)]+[2 for i in range(4)]
nindex=['x'+str(i) for i in range(len(windexd))]
nindexd=[i for i in range(len(windexd))]

index2=pd.MultiIndex(levels=[windex,nindex],
                            codes=[windexd,nindexd]

s2=pd.Series(np.random.randint(3,13,9),index=index2)

s2['b':'a']

https://www.cnblogs.com/kuangkuangduangduang/p/10258654.html

s2.loc['b':'a']