randomise portfolio item grid

Hi,
I’ve searched your forum and found a couple of posts relating to this but nothing has worked.
I am using a portfolio grid on numerous pages. I would like these to display portfolio items randomly. Currently it shows them in the order that is in the back-end.
I tried this code in my functions file but this did not work:
<?
function custom_post_grid_query( $query, $params ) {
$query[‘orderby’] = ‘rand’;
return $query;
}
add_filter( ‘avia_post_grid_query’, ‘custom_post_grid_query’, 10, 2);
?>

Thanks very much :)