admins: kerberos instance

[ms419 at freezone.co.uk]

I am trying to make one instance of a kerberos principal the cyrus 
administrator. I've set the "admins:" field to "username/admin". When I 
try running "cyradm -u username/admin servername", cyradm complains:

cyradm: cannot authenticate to server as user username/admin

Syslog contains the following entry:

badlogin: wum.lat[192.168.179.73] GSSAPI [SASL(-13): authentication 
failure: bad userid authenticated]

What's the problem?

Thanks,

Jack