diff --git a/encoding.c b/encoding.c
index f0411d16ee..6c70c74137 100644
--- a/encoding.c
+++ b/encoding.c
@@ -1207,6 +1207,11 @@ set_default_internal(VALUE klass, VALUE encoding)
  *     LANG=ja
  *       Encoding.locale_charmap  => "eucJP"
  *
+ * The result is higly platform dependent.
+ * So Encoding.find(Encoding.locale_charmap) may cause an error.
+ * If you need some encoding object even for unknown locale,
+ * Encoding.find("locale") can be used.
+ *
  */
 VALUE
 rb_locale_charmap(VALUE klass)