Why this message came when i added user in group?

Hi all,

When I added one user in in this group hhs_gl6 following message got generated.

-bash-3.00$ /usr/local/bin/sudo /usr/sbin/usermod -G hhs_gl6 vivek
UX: /usr/sbin/usermod: hhs_gl6 name should be all lower case or numeric.

However when I cheked the user in /etc/group file, the user vivek was added in hhs_gl6 group.

Please tell me will there be any issue because of this error which occurred?
I don't thing so, what all you say guys ...?

Thanks.
Manali

It is complaining about the underscore character.
Here is why from the doc set: