funzione che stampa un errore

[rocco.mod]

adesso ho fatto un get_num_rows pero mi da errore

        $dbcore = new DB();
        $dbcore->connect();
        $this->resultSet = $dbcore->getnumrows($this->query); 

errore

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in C:\Programmi\Apache Software Foundation\Apache2.2\htdocs\florDataGridV1_1\db_class.php on line 82

    function getnumrows($sql) {
        $result = $this->query($sql);
        $num = mysql_num_rows($result);//<-qui e l errore
        @mysql_free_result($result);
        return $num;
    }