The Newsletter subscription form widget is displayed on the homepage. When a user submits an email address, there are 2 issues:
1) The user is not added to my Subscribers in the WP admin
2) A message ‘undefined’ is displayed to the user on the front end
You should ask who coded the widget to make the fields compatible with Newsletter, they’re not, they have all wrong name. On config panels of newsletter there are example of form codes.