diff --git a/nix/default.nix b/nix/default.nix
index f28abacc859ac5e1dd301259583cfaf5f7f04b27..ba25519912fa66efcfd5e7d4029a62aed9bf3a3d 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -8,7 +8,7 @@ let mydir = builtins.getEnv("PWD");
       selection = "${mydir}/${name}-${ocaml.version}-opam-selection.nix";
     };
      opamPackages =
-      [ "ocamlfind=1.9.3" "zarith" "ocamlgraph" "yojson" "zmq"
+      [ "ocamlfind=1.9.3" "zarith" "ocamlgraph" "yojson=1.7.0" "zmq"
         "ppx_import" "ppx_deriving" "ppx_deriving_yojson"
         "coq=8.13.0" "alt-ergo=2.2.0"
         "why3=1.5.0" "why3-coq=1.5.0"