[Python] In pandas, a groupby followed by boxplot gives KeyError: "None of [Index(['A', 1],...

This very simple script gives error KeyError: "None of [Index(['A', 1], dtype='object')] are in the [index]":

import pandas as pd
import matplotlib.pyplot as plt
L1 = ['A','A','A','A','B','B','B','B']
L2 = [1,1,2,2,1,1,2,2]
V = [9.8,9.9,10,10.1,19.8,19.9,20,20.1]
df = pd.DataFrame({'L1':L1,'L2':L2,'V':V})

print(df)

df.groupby(['L1','L2']).boxplot(column='V')
plt.show()


So my dataframe is:

L1 L2 V
0 A 1 9.8
1 A 1 9.9
2 A 2 10.0
3 A 2 10.1
4 B 1 19.8
5 B 1 19.9
6 B 2 20.0
7 B 2 20.1


and I would expect a plot with four boxplot showing the values V, the labels of boxplots should be A/1, A/2, B/1, B/2.

I had a look at How To Solve KeyError: u"None of [Index([..], dtype='object')] are in the [columns]" but I was not able to fix my error, AI tools are not helping me either.

What am I not understanding?

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