使用PHP 5.4时，Prepared Statement失败

The following statement works perfectly fine under PHP 5.2:

$db = new mysqli($db_host, $db_user, $db_pass, $database);

$sql = $db->prepare('SELECT id, field1, field2, field3 FROM people WHERE email = ? AND pass=?');

$user = strtolower($_POST['user']);
$pass = md5($_POST['pass']);

$sql -> bind_param('ss', $user ,$pass);
$sql -> execute();
$sql -> bind_result($id, $field1, $field2, $field3);

if ($sql -> fetch()) {
  ...
}

However, after upgrading to PHP 5.4 the fetch() fails and gives the following error:

Attempt to read a row while there is no result set associated with the statement

I couldn't find any hints that something has changed regarding the functions I use and the way I use them. I have seen that there was a change for bind_param using arrays in 5.3, but as I'm not using arrays here, I don't think that I'm affected by this change.