[Python] Updating a column with another column's values in python but only NaN values

Hi I'm trying to use the .update() function to update the values of one column (B) with values from another column (C) in the same dataframe. However, I would like to only replace the values in column B that are NaN. I have tried using the overwrite = False argument but I keep getting an error:

df = pd.DataFrame({'A': [1, 2, 3],
'B': [400, np.nan, 600],
'C': [32,54,300]})
df['B'].update(df['C'], overwrite = False)
df

#Output:
#TypeError: update() got an unexpected keyword argument 'overwrite'

#The intended output I'm looking for is this:
A B C
0 1 400.0 32
1 2 54.0 54
2 3 600.0 300


However, when I use the update.() function to update the whole dataset, the overwrite argument works:

df = pd.DataFrame({'A': [1, 2, 3],
'B': [400, np.nan, 600],
'C': [32,54,300]})
new_df = pd.DataFrame({'B': [4, 5, 6],
'C': [7, 8, 9]})
df.update(new_df, overwrite = False)
df

#Output:
A B C
0 1 400.0 32
1 2 5.0 54
2 3 600.0 300


Does the overwrite argument just not work when updating a column with another column in the same dataframe? Would like to check how I might go about updating a column with another column's values but only overwriting values that are NaN. Thank you so much!

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