diff --git a/io.c b/io.c
index 572b9575b1..f988c1b70e 100644
--- a/io.c
+++ b/io.c
@@ -8043,6 +8043,9 @@ rb_io_make_open_file(VALUE obj)
  *    Specifies file open flags as integer.
  *    If +mode+ parameter is given, this parameter will be bitwise-ORed.
  *
+ *  :\external_encoding ::
+ *    External encoding for the IO.
+ *
  *  :\internal_encoding ::
  *    Internal encoding for the IO.  "-" is a synonym for the default internal
  *    encoding.