Error locations incorrect when a deriver is missing

[hcarty]

Compiling the following reports an error with no location. It should probably be on the j in y: j. This is using the version 3.1 or the latest master branch of ppx_deriving_yojson and ppx_deriving 4.2.1.

test.ml

(* Just a comment *)

type j

type t = {x: int; y: j} [@@deriving yojson]

jbuild

(jbuild_version 1)

(executable
  ((name test)
   (preprocess (pps (ppx_deriving_yojson)))
   (libraries (ppx_deriving_yojson.runtime))))

Build error with:

$ jbuilder build test.exe
      ocamlc .test.eobjs/test.{cmi,cmo,cmt} (exit 2)
(cd _build/default && /home/hcarty/.opam/4.06.0/bin/ocamlc.opt -w -40 -g -bin-annot -I .test.eobjs -I /home/hcarty/.opam/4.06.0/lib/biniou -I /home/hcarty/.opam/4.06.0/lib/easy-format -I /home/hcarty/.opam/4.06.0/lib/ppx_deriving -I /home/hcarty/.opam/4.06.0/lib/ppx_deriving_yojson/runtime -I /home/hcarty/.opam/4.06.0/lib/result -I /home/hcarty/.opam/4.06.0/lib/yojson -no-alias-deps -o .test.eobjs/test.cmo -c -impl test.pp.ml)
File "_none_", line 1:
Error: Unbound value j_to_yojson
Hint: Did you mean to_yojson?