# Dune error - Ppx dependency on a non-ppx library **URL:** \ **Category:** Ecosystem\ **Tags:** dune\ **Created:** [June 22, 2020, 3:55pm UTC](https://discuss.ocaml.org/t/dune-error-ppx-dependency-on-a-non-ppx-library/6011 "2020-06-22T15:55:28Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![roddy](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/roddy/32/2261_2.png) [@roddy](https://discuss.ocaml.org/u/roddy)\ **Post date:** [June 22, 2020, 3:55pm UTC](https://discuss.ocaml.org/t/dune-error-ppx-dependency-on-a-non-ppx-library/6011/1 "2020-06-22T15:55:28Z")
I’m getting a strange dune error from trying to compile a project with pre-4.07 OCaml, Dune 2.6 and ppxes that are built with jbuilder: ```auto File "bin/dune", line 6, characters 7-14: 6 | (pps ppx_let))) ^^^^^^^ Error: Ppx dependency on a non-ppx library "ppx_let". If "ppx_let" is in fact a ppx rewriter library, it should have (kind ppx_rewriter) in its dune file. ``` I can’t reproduce it with a minimal example (i.e. where ppx\_let is the only library). It’s on CI [here](https://github.com/roddyyaga/finch/runs/792912734?check_suite_focus=true#step:7:14) but I also get it locally. Does anyone have any ideas what might be causing it? ---
**Author:** ![rgrinberg](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/rgrinberg/32/40_2.png) [@rgrinberg](https://discuss.ocaml.org/u/rgrinberg)\ **Post date:** [June 23, 2020, 1:22am UTC](https://discuss.ocaml.org/t/dune-error-ppx-dependency-on-a-non-ppx-library/6011/2 "2020-06-23T01:22:29Z")
I think it’s just an issue with you using an extremely old version of `ppx_let`. The version of `ppx_let` that you’re using is still on jbuild files, which don’t generate the correct metadata. In particular, the (installed in `dune-package` files) metadata doesn’t declare that ppx\_let is a rewriter. ---
**Author:** ![roddy](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/roddy/32/2261_2.png) [@roddy](https://discuss.ocaml.org/u/roddy)\ **Post date:** [June 23, 2020, 7:28pm UTC](https://discuss.ocaml.org/t/dune-error-ppx-dependency-on-a-non-ppx-library/6011/3 "2020-06-23T19:28:44Z")
Thanks! I was a bit confused because the jbuild file did seem to specify it as a ppx rewriter.