Creating an “admin only” meta box with WPAlchemy. Getting a fatal error on front-end when using current_user_can

try the following:

$custom_admin_mb = new WPAlchemy_MetaBox(array(
    'id' => '_custom_admin_meta',
    'title' => 'Admin only',
    'template' => get_stylesheet_directory() . '/custom/admin_meta.php',
    'output_filter' => 'my_output_filter',
));

function my_output_filter() {
    if (current_user_can('administrator')) return true;
    return false;
}