Strange Error on working PHP

[Entourage]

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in www/htdocs/test/admin/db_mysql.php on line 154

PHP Code:

$currenthits=$DB_site->query_first("SELECT hits FROM main WHERE id=1");
$negertje=$currenthits['hits'] + 1;
$DB_site->query_first("UPDATE main SET hits=$negertje WHERE id=1"); 


TABLE main (
hits int(10) NOT NULL default '0',
id tinyint(4) NOT NULL default '1'
) TYPE=MyISAM;

It all works but gives that stupid error :/ why o why?