portupgrade fails: no locking avaiable

[llx]

i've a nfs server (FreeBSD 4.9) which exports the ports tree via NFS.
on the client machine (FreeBSD 5.1) portupgrade fails:  

 >  checking configure summary... ERROR: No locking available. Running Samba  
     would be unsafe
 > configure: error: summary failure. Aborting config

why does this happen? i thought that FreeBSD 5.x supports nfslocking as a 
client. on the server is started rpc.lockd and rpc.statd (see output rpcinfo 
below). 

cheers
llx

ipcinfo -p <SERVER> 

sh-2.05b$ rpcinfo -p frigi
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100004    1   udp   1021  ypserv
    100004    2   udp   1021  ypserv
    100004    1   tcp   1023  ypserv
    100004    2   tcp   1023  ypserv
    100009    1   udp   1011  yppasswdd
    100009    1   tcp   1022  yppasswdd
    100005    3   udp   1008  mountd
    100005    3   tcp   1021  mountd
    100005    1   udp   1008  mountd
    100005    1   tcp   1021  mountd
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100003    2   tcp   2049  nfs
    100003    3   tcp   2049  nfs
    100021    1   udp    994  nlockmgr
    100021    3   udp    994  nlockmgr
    100021    4   udp    994  nlockmgr
    100021    1   tcp   1020  nlockmgr
    100021    3   tcp   1020  nlockmgr
    100021    4   tcp   1020  nlockmgr
    100024    1   udp    986  status
    100024    1   tcp   1019  status