Using GridFieldExpandableForm forbidden in ModelAdmin?

[JustinTBrown]

I'm attempting to use ExpandableForm on my ModelAdmin's managed model. Not on a relation of managed model which the current documentation gives an example for. When I click on a record's row, I get a popup error, "Forbidden". Here's the getEditForm in the model admin:

  public function getEditForm($id = null, $fields = null) {
    $form = parent::getEditForm($id, $fields);
    $gridFieldName = $this->sanitiseClassName($this->modelClass);
    $gridField = $form->Fields()->fieldByName($gridFieldName);

    $expandedFields = new FieldList(array(
      new TextField('Content'),
    ));
    $gridField->getConfig()->addComponent(new GridFieldExpandableForm($expandedFields));

    return $form;
  }

And here's what I'm getting in my browser's console:

Artist/EditForm/field/Artist/expand/31 Failed to load resource: the server responded with a status of 403 (Forbidden)

Any ideas what's going wrong?