activeadmin: Changing config.authentication_keys to :login breaks Active Admin login

I have setup my application with Active Admin. I have to say it’s extremely simple to get running! I have set Devise up in my application before I installed and setup Active Admin. I wanted to allow users to login with either username or password so I changed config.authentication_keys = [ :email ] to config.authentication_keys = [ :login ]. Users can login with either their username or password. Going back into active admin, I am unable to login. It just shows

NoMethodError in Active_admin/devise/sessions#new
undefined method `login' for #<AdminUser:0x007fe5d62bc730>

Changing the config.authentication_keys = [ :login ] back to config.authentication_keys = [ :email ], I am able to login to Active Admin no problems.