When /etc/group contains a line such as
chkgrp issues the following warning:
/usr/sbin/chkgrp: /etc/group: line 34: strtoul failed
/etc/group is fine
where line 34 is the one containing the NIS entry. This occurs regardless of whether the NIS entry is the last line in /etc/group.
Add the line
to /etc/group on a NIS client and run /usr/sbin/chkgrp (or wait for periodic to do that).
This has been fixed in head in 2014/2015 so should be in all supported releases now.