pnfs MDS mount point clinet cannot mount

Hi all,
I am glad to use your nfs-utils for pnfs server.
Then I have a question when I build pnfs environment.
I use nfs-utils-3-3-rc5 and linux-4.1.1 as MDS. Other five DS use two disk respectively iSCSI target.
My client have no upgrade nfs-utils and linux, just Fedora 22. The problem is when my client mounts the pnfs MDS by command:

# mount -t nfs -o vers=4.1 MDS:/export /pnfs/
The system appears message:
# mount.nfs: mounting MDS:/export failed, reason given by server: No such
file or directory
But is successfully with mount MDS:/
# mount -t nfs -o vers=4.1 MDS:/ /pnfs/
It means that I cannot mount any mount point but the slash of MDS.
I see google seldom people meet the question and no one answer that.
Therefore, I ask to nfs-utils developer.
Sorry to bother you and thank you read the mail.

Regards,
Paul