users-admin has problem when username contains a dot

Binary package hint: gnome-system-tools

on Ubuntu Maverick,
when trying to create a user, I get a windows which says that I can use special characters, such as: '.' '-' '_'
In other words, the tools says that I am allowed to use the dot, but this is slightly wrong.
The dot is used by chown command, as an alternative to colon, to separate user and group: "chown user.group <file>" is equivalent to "chown user:group <file>"
The problem is that users-admin is not throwing out the correct error, and it's only saying: "The configuration could not be saved. You are not allowed to modify the system configuration."
But if you remove the dots from the username, you'll be "authorized" and you can create users....

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: gnome-system-tools 2.32.0-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.35-23.40-generic 2.6.35.7
Uname: Linux 2.6.35-23-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
Date: Sun Nov 21 19:30:56 2010
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release i386 (20101007)
ProcEnviron:
 PATH=(custom, no user)
 LANG=it_IT.utf8
 SHELL=/bin/bash
SourcePackage: gnome-system-tools