TJSDocument not working properly with Module Sub-Types (Data Models)

export default class MyCustomDataModel extends foundry.abstract.TypeDataModel {
  static defineSchema() {
    const fields = foundry.data.fields;

    return {
      customField: new fields.StringField({}),
    }
  }

  updateField(value) {
    this.updateSource({customField: value});
  }
}

  updateField(value) {
    this.parent.update({"system.customField": value});
  }