Pandas模块的为df添加新的column时值为Nan

# 我想在df1中添加一列，然后groupby，但是我按照下面方法添加总是Nan
df1 = pd.DataFrame({'a':[1, 2, 3], 'b': [11, 33, 44], 'c': [123,456, 788]}, index=list('ABC'))   
""" 
   a   b    c
A  1  11  123
B  2  33  456
C  3  44  788
"""

df2 = pd.DataFrame(np.array([66, 55, 44]).reshape((3, 1)))  
df1['d'] = df2
print(df1)
"""
   a   b    c   d
A  1  11  123 NaN
B  2  33  456 NaN
C  3  44  788 NaN
"""

# 原因在于索引
df2 = pd.DataFrame(np.array([66, 55, 44]).reshape((3, 1)), columns=list('ABC'))  # 注意添加时候的索引
df1['d'] = df2
print(df1)
"""
   a   b    c   d
A  1  11  123  66
B  2  33  456  55
C  3  44  788  44
"""