如何在多索引DataFrame的子集中添加pd.Series？

subset = df.loc[(level1_label, level2_label)]

df.loc[(level1_label, level2_label), 'column_name'] = series

import pandas as pd

# 创建一个多索引DataFrame
data = {'A': [1, 2, 3, 4],
        'B': [5, 6, 7, 8]}
index = pd.MultiIndex.from_tuples([('a', 'x'), ('a', 'y'), ('b', 'x'), ('b', 'y')], names=['level1', 'level2'])
df = pd.DataFrame(data, index=index)

# 选择子集并添加pd.Series
subset = df.loc[('a', 'x')]
series = pd.Series([9, 10], index=['A', 'B'])
df.loc[('a', 'x'), 'C'] = series

print(df)

            A  B     C
level1 level2         
a      x     1  5   NaN
       y     2  6   NaN
b      x     3  7   NaN
       y     4  8   NaN
a      x     9  10  9.0