Creating a volume - Getting error

When I go to the Create Volume web page I configure as follows:

Name: calibre-ebooks
Driver: Local
Driver Options: NAME: mountpoint VALUE: /volume1/ebooks

I then push the button to create the volume and get the following error

Create calibre-ebooks, invalid option key, “mountpoint”

What option should i use. I literally tells me to use mountpoint in the example. And I can’t find any documentation that tells me what the valid NAME values are.

I’m running Docker-ce on an Asustor NAS.
Engine Details

Version 18.06.3-ce (API: 1.38)
Root directory /volume1/.@plugins/AppCentral/docker-ce/docker_lib
Storage Driver overlay2
Logging Driver json-file
Volume Plugins local
Network Plugins bridge, host, macvlan, null, overlay

I have attached a screenshot of the error. Any assistance would be awesome.