Changing password for Administror@vsphere.local in vRA 7.01

We have a security requirement to change the password of all local users.  Using PowerVRA I can successfully change the password of other users (e.g. configurationadmin) but when I attempt to change the password of administrator@vsphere.local I get the following error from the REST call

Invoke-RestMethod : {"errors":[{"code":400,"message":"Cannot set a password for this user.","moreInfoUrl":null}]}

Does anybody know if / how this password can be set/changed?