Python Error 'NoneType' Object [on hold]

I have the following code

br.form['session[Client_username]'] = username
br.form['session[Client_login_mode]'] = password

It's meant to log me into a website automatically. Client_username and Client_login_mode are the form names.

I get an error 'NoneType' object does not support item assignment

How do I fix this?