diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 88c58008f..7a081586d 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -1678,8 +1678,8 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "initialize_copy", io_init_copy, MRB_ARGS_REQ(1));
   mrb_define_method(mrb, io, "_check_readable", io_check_readable, MRB_ARGS_NONE());
   mrb_define_method(mrb, io, "isatty",     io_isatty,     MRB_ARGS_NONE());
-  mrb_define_method(mrb, io, "sync",       io_sync,       MRB_ARGS_NONE());
-  mrb_define_method(mrb, io, "sync=",      io_set_sync,   MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, io, "sync",       io_sync,       MRB_ARGS_NONE());   /* 15.2.20.5.18 */
+  mrb_define_method(mrb, io, "sync=",      io_set_sync,   MRB_ARGS_REQ(1));   /* 15.2.20.5.19 */
   mrb_define_method(mrb, io, "sysread",    io_sysread,    MRB_ARGS_ARG(1,1));
   mrb_define_method(mrb, io, "sysseek",    io_sysseek,    MRB_ARGS_ARG(1,1));
   mrb_define_method(mrb, io, "syswrite",   io_syswrite,   MRB_ARGS_REQ(1));