mount.nfs: an incorrect mount option was specified

Hit the same issue today. I stumbled upon option nfsvers when searching for an explanation. Mounting worked with nfsvers=3 and nfsvers=4. I'd be glad for more detailed explanation though.

root@localhost:~# uname -rm
4.1.15 armv7l
root@localhost:~# mount -t nfs 10.0.0.5:/srv/nfs tmp
mount.nfs: an incorrect mount option was specified
root@localhost:~# mount -t nfs -o nfsvers=1 10.0.0.5:/srv/nfs tmp
mount.nfs: mount system call failed
root@localhost:~# mount -t nfs -o nfsvers=2 10.0.0.5:/srv/nfs tmp
mount.nfs: requested NFS version or transport protocol is not supported
root@localhost:~# mount -t nfs -o nfsvers=3 10.0.0.5:/srv/nfs tmp
root@localhost:~# mount -t nfs -o nfsvers=4 10.0.0.5:/srv/nfs tmp
root@localhost:~#

Last two commands succeeded.

I found this issue today on Tiny Core Linux, turned out to be the fact that the nfs client service hadn't been started. Once I started that service (/usr/local/etc/init.d/nfs-client start) the NFS export mounted without issue.