Select Query Error

Hi everyone, i have a problem with a select query, 

Code:

$where = "SELECT FORMAT(SUM(payed), 2) AS payedAmount FROM client_credit WHERE creditStatus = 2 AND idperson = '"
           .$post_data['idperson']. "' AND contractDate
       BETWEEN '"
           . $startDate . "' AND '" . $endDate . "'";
$query = $this->db->query($where);
       if ($query->num_rows() > 0) {
           return $query->result();
       } else {
           return null;
       }

But i get a boolean result, $query = false, and when i execute the same query in mysql console i get the right result, but here i get boolean.

Thank you.