Missing argument 2 for wpdb::prepare()

add_filter('posts_orderby_request', 'wpjam_search_orderby_filter');
	function wpjam_search_orderby_filter($orderby = ''){
		if(is_search()){
			global $wpdb;
			$keyword = $wpdb->prepare($_REQUEST['s']);
			return "((CASE WHEN {$wpdb->posts}.post_title LIKE '%{$keyword}%' THEN 2 ELSE 0 END) + (CASE WHEN {$wpdb->posts}.post_content LIKE '%{$keyword}%' THEN 1 ELSE 0 END)) DESC, {$wpdb->posts}.post_modified DESC, 	{$wpdb->posts}.ID ASC";
		}else{
			return $orderby;
		}
	}

Warning: Missing argument 2 for wpdb::prepare(),just this $keyword = $wpdb->prepare($_REQUEST['s']);