rpc.yppasswdd failed in FreeBSD 5.1

[Lin, Tsung Ching]

hello, 
I setup a NIS Master in FreeBSD 5.1 release, and it works.
I also startup rpc.yppasswdd.
When I wnat to change a user's passwd using yppasswd, it failed.

root# passwd testuser
Changing NIS password for testuser
Old Password:
New Password:
Retype New Password:
passwd: pam_chauthtok(): error in service module
Jun 13 22:22:24 abcb passwd: in pam_sm_chauthtok(): yppasswd_local():
failed to connect to rpc.yppasswdd: xxxx.xxx.xxx.xxx: RPC: Program not
registered

I had set hosts.allow for rpcbind and ypserv to allow accessing.
please help me to solve this.
thank you.

Tsung Ching Lin