Errors when setting permissions via REST using new v3 endpoint... bug or user error?

I am getting errors using the new POST /api.ashx/v3/permissions endpoint.

Documentation: https://community.telligent.com/community/11/w/api-documentation/67092/set-permission-v3-rest-endpoint

Here is an example call...

curl https://mysite.net/api.ashx/v3/permissions.json \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -H 'Rest-User-Token: ENCODED_TOKEN' \
  -d 'IsAllowed=True&RoleId=1889&PermissionId=fd61ae45-30c5-4b04-92c4-9c6ec2fe1c90&GroupId=301'

The RoleID and GroupId have been triple checked, and I do get results when calling the GET /api.ashx/v3/permissions endpoint with these values.

The PermissionId "fd61ae45-30c5-4b04-92c4-9c6ec2fe1c90" is for "Read Group".

The result is the following message...

{
    "Permission": null,
    "Errors": [
        "An unexpected error prevented your requested action from completing. The error has been logged for the administrator to review."
    ]
}

And the exception log provides this message...

System.InvalidOperationException: Nullable object must have a value.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at Telligent.Evolution.Rest.Implementation.Resources.PermissionsRestRoutePlugin.SetPermission(IRestRequest request)

The error indicates a missing value, but the only other documented parameter is ApplicationId, and the API still throws the same error even when a value is provided for that (along with an Application specific permissionID).

Any ideas?  Is this a bug, or a documentation error, or am I missing something?

Also... I noticed that there doesn't seem to be a Widget API equivalent to this API.  Is that something that is planned?

Thanks.