выводит ошибку Fatal error: Call to a member function fetch_assoc() on a non-object in Z:\home\veterinar.com\www\obzor.php on line 5

<?php
require_once('header.php');
function printResultSet($result_set) {
  echo "Количество записей:".$result_set->num_rows."</br>";
  while (($row = $result_set->fetch_assoc()) != false) 
  {
    print_r($row);
    echo"<br/>";
  }
  echo "---------------<br/>";
}
$a = intval($_GET['Id_city']);
$mysqli= new mysqli ("localhost", "root", "", "vetbase");
$mysqli->query("SETNAMES 'utf-8'");
$result_set = $mysqli->query("SELECT `City`, `Clinic_name`, `Clinic_address`, `Clinic_contacts` FROM `allclinics` WHERE id = {$a} ");
 
$mysqli->close();
?>
<form class="content" method="post">
<?php printResultSet($result_set); ?>  
  </form>;

$row = $result_set->fetch_assoc($result_set)

$mysqli->set_charset('utf8');

$row = $result_set->fetch_assoc()

$row = $mysqli->fetch_assoc($result_set)