Fatal Error:call to a member function Fetch_array () on a non-object in the line, how to handle

Fatal Error:call to a member function Fetch_array () on a non-object in
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 = $sql->fetch_array (mysql_both); 62 rows
if ($res!=null) {

Session_register ("Admin_user");
Session_register ("Admin_pass");
echo "";
}else{
echo "";
}
}

?>
------to solve the idea----------------------
echo $conn->error; on line 62 Look

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