Impossible d'effectuer la requête

Voici la requête :
SELECT cat_name FROM an_categ WHERE tb_id='168' and cat_id=

Voici le message d'erreur renvoyé par la base de données :
1064 : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /homez.55/maisondh/www/annonces/annuaire.php on line 51