[2.1]: Can not add a "can not register" ban, for a username that is not already registered

[LexArma]

Basic Information

Trying to add a partial ban for a username, to stop said username from registering, ends up in the error message

"The following error or errors occurred while saving or editing the ban Member name not found"

Steps to reproduce

1. Create a partial ban, select can not register, and ban on username
2. Give any nonexisting username
3. See the results

Expected result

A ban should be added, and the given username should not be available to be registered.

Actual result

You can not even create the ban.

Version/Git revision

Tested on 2.1.4

Additional Information

This exact same ban setup does work with emails and IP addresses, why not usernames? This could be very handy sometimes, if it also allowed wildcards like emails and IP addresses do. Discussion in https://www.simplemachines.org/community/index.php?topic=590247