Autosubscribed notifications don't go out

I have enabled autosubscription of notifications for a given client by adding a user to the field labeled "Auto-subscribe:" on the client configuration page. When I enter a support ticket for the client, a notification is sent to me (as the ticket owner), but nothing is set to the email address of the user entered for autosubscription. Further, the user name set to autosubscribe for the client does not appear in the "Subscribed" box of the "Notifications" section on the ticket.

In the Drupal log, an entry for the support module with the message "User (0) has no email address." appears. I have found that even if I remove the autosubscribe user from the client form, the log message will still appear when new tickets are entered.

I have also found that adding the user subscription manually to a ticket from the "Subscribed" box of the "Notifications" section on the ticket does not cause notifications to be sent out.

Any advice would be appreciated.