Call to a member function fetch_assoc() on a non-object

[J_cole]

代码如下：
require_once '../pd_inc/pd_mysqlHelper.class.php';
$prov = array();
$my = new mysqlHelper();
$my->execute_dml('use city');
$query = $my->execute_dql("call pd_seek_city(1, 0, null, null);");
while($res = $query->fetch_assoc()){
$prov[] = $res;
}
?>
//菜鸟萌新，请大神多多指教，拜托了

[xuzuning]

查询执行失败 execute_dql 返回的不是对象