Error message "Could not add users" when I try to send an invite to a user without a bitbucket accnt

[scout3801]

I'm trying to add a user via their email address, but it fails.

When I add users who have bitbutcket accounts (as I have), that succeeds.

I'm guessing there's a connection there, but I'm not sure.  Should I only be able to send invites to people who already have atlassian related (ie bitbucket) accounts?