Boilerplate theme - Notice : Uninitialized string offset: 0...

To template.php in Boilerplate :

function mon_vintage_id_safe($string) {
// Replace with dashes anything that isn't A-Z, numbers, dashes, or underscores.
$string = strtolower(preg_replace('/[^a-zA-Z0-9_-]+/', '-', $string));
// If the first character is not a-z, add 'n' in front.
if (!ctype_lower($string{0})) { // Don't use ctype_alpha since its locale aware.
$string = 'id'. $string;
}
return $string;
}

Why on my page I have the warning?:

Notice : Uninitialized string offset: 0 dans mon_vintage_id_safe() (ligne 150 dans C:\wamp\www\respecte-moi.fr\sites\all\themes\boilerplate\template.php).

Thank you for your help