[Vala] KeyFile bug or doc error

[Nor Jaidi Tuah <norjaidi tuah ubd edu bn>]

valac version: 0.7.10

The interface in KeyFile class (in glib-2.0.vapi):

  public string get_locale_string (string group_name, string key, 
  string locale) throws KeyFileError 

The documentation
http://references.valadoc.org/glib-2.0/GLib.KeyFile.get_locale_string.html says:

  If locale is null then the current locale is assumed.

Should the vapi declares "string? locale" (nullable)?
Or, should the doc be corrected?

hand
Nor Jaidi Tuah