ReasonML Warning: Unimplemented primitive used

If you run into the issue Warning: Unimplemented primitive used:put or whatever put is for you, it's probably because you forgot to include the [@bs.module "<library>"] or any other FFI before your wrapping.

For example:

external putJson: (string, Js.Json.t) => request = "put";

which gave me the error above ^. The fix?

[@bs.module "superagent"] external putJson: (string, Js.Json.t) => request = "put";

At least that was the case for me. (: