[Plugin: Subscribe to Comments] Need an error for no email on file

I began using Facebook Connect (which I think will become very popular soon), which creates a user profile leaving the e-mail address field blank. Whenever a user comments on a post and checks to receive updates via email they get nothing. Is there a way to notify them we do not have an e-mail on file for them? My website: http://ourconvo.com

Thanks.

http://wordpress.org/extend/plugins/subscribe-to-comments/