PHP & MySQLi Fatal error: Call to a member function bind_param() on a non-object

I'm facing this problem and I've searching for hours
now..
I can't see any problem in this piece of code... What's wrong
here?

db.php:

$mysqli
            = mysqli_connect('localhost', 'root', '',
            'site');

if(mysqli_connect_errno()) 
 {
 echo
            "Erro BD";
 exit();

            }
?>

update.php:

include
            'db.php';

$zzz = '';
$email =
            'asdsaf@gmail.com';

$ins = $mysqli->prepare("update USER set
            cod_valida =? where email =?");
$ins->bind_param('ss', $zzz,
            $email);

 if ($result =
            $ins->execute()){


 $ins->free_result();

            echo "It works";
 }
 else
 {
 echo
            "Error";

            }
$ins->close();
$mysqli->close();

Is
there anyway to echo the query I'm trying to send or the mysql
error?

Thank you for your help instead of
downvote my thread since I already checked everything and found no
answer.