Error message when adding a reminder as anonymous user

When I'm testing adding a reminder as an anonymous user with this module, it displays (and logs) the following error message:

Notice: Undefined index: send-now in _anon_datereminder_form_submit_user() (line 384 of /server/path/to/drupal/sites/all/modules/anon_datereminder/anon_datereminder.module).

This error is not generated when I'm adding reminders as User 1 (where I see the checkbox for "Send test email"). Running Drupal core 7.50.

If this is something that can be avoided through better config, please advise.