showmount returns error

I have two Mac's on my LAN. One runs 10.9.5 and the other runs OS X 10.10. Both show the same response (see below) when I try to run showmount.

showmount: Cannot retrieve info from host: localhost: RPC: Port mapper failure - RPC: Remote system error

The server on my LAN is Ubuntu (NFS). And both Mac's are connecting to the NFS shares on the server without any problems and all other networking functionality is fine (Internet, printing, etc.).

And the showmount command works from other (non-Mac) clients on the network. I would like to get showmount to work from Mac OS X. Any suggestions?