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.