If Errn$ is called, and errn is 0, it causes "Message not found undefined" to appear instead.
Print ERRN,ERR$ End
0 Message not found undefined
The ERRN prints properly, but ERR$ returns a bogus message.
I think it would be appropriate to return an empty string when ERRN is 0.
Tested in newset version from git as of 11/27/2022 (macOS).