Unfortunately this no longer works in 4.02, since caml_alloc_tuple now ([05100e5][caml-alloc-val-unit]) initializes fields to `Val_unit`, not to `0`.

[caml-alloc-val-unit]: https://github.com/ocaml/ocaml/commit/05100e5#diff-f940572c32e55a7e40e7f693deea4c7bR42