Missing argument 2 for wpdb::prepare()

I updated wordpress to 3.5.There is one error occurs:

Warning: Missing argument 2 for wpdb::prepare(), called in /home/... and defined in /home/cameraqu/public_html/wp-includes/wp-db.php on line 990
4 items)

My code:

function countOrders () {
global $ wpdb;
$ ordersTable = 'wp_orders';
$ count = $ wpdb-> get_var ($ wpdb-> prepare ("SELECT COUNT (*) FROM $ ordersTable;"));
return $ count;
}

It error line $ count = $ wpdb-> get_var ….
helpppppppppppppppppppppp