Clear `__classname__` of duped class/module; ref
@@ -282,7 +282,9 @@ init_copy(mrb_state *mrb, mrb_value dest, mrb_value obj)
case MRB_TT_CLASS:
case MRB_TT_MODULE:
copy_class(mrb, dest, obj);
- /* fall through */
+ mrb_iv_copy(mrb, dest, obj);
+ mrb_iv_remove(mrb, dest, mrb_intern_lit(mrb, "__classname__"));
+ break;
case MRB_TT_OBJECT:
case MRB_TT_SCLASS:
case MRB_TT_HASH:
