Fatal error: Call to a member function fetch_array() on a non-object in 62 line,该如何处理

Fatal error: Call to a member function fetch_array() on a non-object in 62 line
 if($_POST[admin_user]!="" || $_POST[admin_pass]!="")
  {
   $conn=new mysqli("localhost","root","db_wlgl);
    $conn->query("set names gb2312");   
    $admin_user=$_POST[admin_user];
    $admin_pass=($_POST[admin_pass]);
    $sql=$conn->query("call admin_regs('".$admin_user."','".$admin_pass."')");
    $res=$sql->fetch_array(MYSQL_BOTH);  //62行
  if($res!=NULL){

session_register("admin_user");
session_register("admin_pass");
   echo "<script>alert('管理员登录成功!');window.location.href='indexs.php';</script>";
}else{
    echo "<script>alert('管理员登录失败!');</script>";   
 }
}

?>
------解决思路----------------------
在62行上面 echo $conn->error; 看看