Nagios authentication token error

Hello,
First time on the forum, I have just tried to install Nagios-2.9 and when I try to start the service it says

root@tardis:/usr/local/nagios# /etc/init.d/nagios start
Starting nagios:You are required to change your password immediately (root enforced)
su: Authentication token is no longer valid; new one required.
(Ignored)
done.
root@tardis:/usr/local/nagios#

Can anyone please advise me what the problem is. This is the first time I have seen
this error occuring. I have installed previous versions of Nagios, which were successful.

Thanks

Patrick

Fixed the problem,
Simply did passwd nagios and the error disappeared.
Cheers

Patrick