[Python] Update a dataframe by dataframes with NaN values

I try to update a DataFrame

df1 = pd.DataFrame(data = {'A' : [1,2,3,4], 'B' : [5,6,7,8]})


by another DataFrame

df2 = pd.DataFrame(data = {'B' : [9, np.nan, 11, np.nan]}).


Now, my aim is to update df1 by df2 and overwrite all values (NaN values too) using

df1.update(df2)


In contrast with the common usage it's important to me to get the NaN values finally in df1. But as far as I see the update returns

>>> df1
A B
0 1 9
1 2 6
2 3 11
3 4 8


Is there a way to get

>>> df1
A B
0 1 9
1 2 NaN
2 3 11
3 4 NaN


without building df1 manually?

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