Passed by reference warning in aperture_preprocess_page()

Using the latest versions of Drupal and Aperture, even after flushing all caches, this message is displayed:

Strict warning: Only variables should be passed by reference in aperture_preprocess_page() (line 21 of D:\_x\Drupal_7_dev\sites\all\themes\aperture\template.php).

The server is running PHP 5.2.5, so that's up-to-date.

The line of code is:

    $vars['primary'] = '<div id="primary-menu">' . drupal_render(menu_tree_output($tree)) . '</div>';

Changing that line to the following, resolves the problem:

    $menu_tree_output = menu_tree_output($tree);
    $vars['primary'] = '<div id="primary-menu">' . drupal_render($menu_tree_output) . '</div>';