Vague feedback when attempting to send email who does not have an email address

Confluence administration allows you to create a user without an email address.
When you attempt to share a page with a user that doesn't have an email address, you get the feedback below.

I am not sure if the problem here is

• the vague error message
• the fact that we can create a user without an email address (and that when you edit the user, email address is a mandatory field)

Reproduced on

Environment

Stable

Browser

IE8
IE9
FF7
Chrome