root login via Kerberos5 - "User not known to the underlying authentication module" - why?

[Wendy Lin]

I am trying to allow user root (uid=0) to be authenticated via
Kerberos5 at login time, too, but if I do I get a "User not known to
the underlying authentication module" error and login is refused.

OS is Suse 13.1

pam config is:
grep -r krb5 /etc/pam.d/
/etc/pam.d/common-password-pc:password  sufficient      pam_krb5.so
/etc/pam.d/common-account-pc:account    required        pam_krb5.so
 use_first_pass
/etc/pam.d/common-auth-pc:auth  sufficient      pam_krb5.so     use_first_pass
/etc/pam.d/common-session-pc:session    optional        pam_krb5.so

What am I doing wrong?

Wendy