TCP NFS w/o -T => "NFS Portmap: RPC: Program not registered"

I got that message when trying to mount an NFS share from a 10.4.1 client. man mount_nfs on the client claimed that the error meant the server was not running mountd. Wrong: it was, and I could mount from other clients. The server was configured to serve NFS over TCP only. Using mount_nfs -T solved the problem. FWIW the server was running 10.3.9.