Trigger error in overlay.module on node save

I'm using this excellent module to enable clients to add/edit nodes using the overlay. However, upon saving a newly created module I get this error message:

Fatal error: Unsupported operand types in C:\wamp\www\drupal\modules\overlay.module on line 238

/**
 * Implements hook_drupal_goto_alter().
 *
 * If the current page request is inside the overlay, add ?render=overlay to
 * the new path, so that it appears correctly inside the overlay.
 *
 * @see overlay_get_mode()
 */
function overlay_drupal_goto_alter(&$path, &$options, &$http_response_code) {
dsm($options);
  if (overlay_get_mode() == 'child') {
    if (isset($options['query'])) {
      $options['query'] += array('render' => 'overlay'); <--- this is line 238
    }
    else {
      $options['query'] = array('render' => 'overlay');
    }
  }
}

When I add a dsm($options) I get three arrays after the save (and after the error has been shown). The first one is empty, the next two look like this:

array('query' => '', 'fragment' => '')

i.e. to index keys but no values.

Now, if I change the operand type from "+=" to "="... the error message disappears, the empty array and ONE of the two arrays with an empty array.

I don't know what's wrong. Perhaps it's that $options is passed by reference... having a dsm($options) at the end of the function shows that it's the first and totally empty $options array instance that is set with render=>overlay.