Pyhton script problem in Power BI.

Hi I have a table already imported in Power BI Desktop with

A         B

'14'     '10'

'12'      null

I was trying to transform that using Python Script in power BI.  I was trying to use the  Dataframe.fillna(value) function in the script that works fine and apparently changes the all the null in the table to the value.

But when I try to change the null value only in one of the column ( in this case in the column 'B') the python script returns me the same dataset. For example, this is the code I am trying to run 

# 'dataset' holds the input data for this script

import pandas as pd

final = dataset['B'].fillna('0')

 above script gives me back the dataset, not the transformed final one as I expected.