Mx classes not recognized by ClutterScript

Mx classes seem to be not registered after import. When ClutterScript tries to load a JSON file with an Mx object, it prints an error message and the object is missing from the object list. However when any (even empty) MxStyle is loaded before, ClutterScript recognizes Mx classes correcly.

GJS script showing the problem:

#!/usr/bin/env gjs
/*
 * Load a MxButton from JSON.
 */

const Clutter = imports.gi.Clutter;
const Mx = imports.gi.Mx;

CSS_DATA = "MxButton {}\n";
JSON_DATA =
  '[{"type": "ClutterStage", "id": "stage", "children": [{ "type": "MxButton", "label": "click me", "id": "panic"}]}]';

Clutter.init(null);

// Uncomment this to load JSON correctly
//Mx.Style.get_default().load_from_data("dummy.css", CSS_DATA);

let script = new Clutter.Script();
script.load_from_data(JSON_DATA, JSON_DATA.length)

objects = script.list_objects()
for (var i = 0; i < objects.length; ++i) {
  print(objects[i]);
}

//Uncomment this to show the loaded stage
/*
let stage = script.get_object("stage");
stage.connect("destroy", Clutter.main_quit);

stage.show();

Clutter.main();
*/

[djdeath]

Could it be because the mx library isn't loaded yet, so ClutterScript can't find them?

Yes, referencing to Mx in other ways works too. So it is an undocumented GI repository importer feature?