Pandas dataframe timedelta is giving exceptions

Question

I am trying to get the next month first date based on billDate in a dataframe.

I did this:

import pandas as pd
import datetime
from datetime import timedelta
dt = pd.to_datetime('15/4/2019', errors='coerce')
print(dt)
print((dt.replace(day=1) + datetime.timedelta(days=32)).replace(day=1))

It is working perfectly, and the output is :

2019-04-15 00:00:00
2019-05-01 00:00:00

Now, I am applying same logic in my dataframe in the below code

df[comNewColName] = (pd.to_datetime(df['billDate'], errors='coerce').replace(day=1) + datetime.timedelta(days=32)).replace(day=1)

But I am getting error like this:

---> 69                 df[comNewColName] = (pd.to_datetime(df['billDate'], errors='coerce').replace(day=1) + datetime.timedelta(days=32)).replace(day=1)
     70                 '''print(df[['billDate']])'''
     71                 '''df = df.assign(Product=lambda x: (x['Field_1'] * x['Field_2'] * x['Field_3']))'''

TypeError: replace() got an unexpected keyword argument 'day'