Lithium Custom Settings - Unable to SET as user

We have a custom setting called user.data (Lithium have setup for us) that is scoped to wcu (world, community, user).

We are trying to SET this custom setting using AJAX - the user account has GRANT for Make REST API calls with modify access

Code

<@liaAddScript>
;(function($) {
   $.post("/restapi/vc/users/self/settings/name/user.data/set", {value: "my user data"}).done(function(data) {
       alert(data);
   }).fail(function() {
       alert("error");
   }).always(function() {
       alert("finished");
   });
})(LITHIUM.jQuery);
</@liaAddScript>

Response

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<response status="error">
<error code="303">
<message>
     User[uid=2369] cannot set field &apos;user.data&apos; on object User[uid=2369]
</message>
</error>
</response>