Or should that be s..t fixed?
Anyway, I have fixed the bug that caused the problem in two places. It is now impossible to enter a blank email address via the front end screens, and its also impossible (in case anyone feels like getting clever and calling the thing directly) fixed the back end such that it will not accept blank email addresses when searching for users, since it doesn't make much sense to return these to anyone anyway!
Again, I apologise. This should never have happened and it should never happen again...