diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index e1c81f68f..020847de2 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -63,6 +63,7 @@ jobs:
         run: |
           opam depext conf-jq --yes # opam depext bug
           opam pin menhirLib 20201216 --no-action
+          opam pin ocaml-index.1.0 https://github.com/voodoos/ocaml-index.git --ignore-pin-depends --ignore-constraints-on=merlin-lib
           opam install --yes ppx_string ppx_compare
           opam install . --deps-only --with-test --yes