diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 07edabc48..728fb9db2 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -11,13 +11,18 @@ on:
     branches: [ master ]
 
 jobs:
-#  nix:
-#    runs-on: ubuntu-latest
-#
-#    steps:
-#    - uses: actions/checkout@v2
-#    - uses: cachix/install-nix-action@v10
-#    - run: nix-build
+  nix:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - uses: cachix/install-nix-action@v12
+    - uses: cachix/cachix-action@v8
+      with:
+        name: coq
+    - run: nix-build
 
   opam:
     runs-on: ubuntu-latest
diff --git a/default.nix b/default.nix
index 511ad8dc0..f00cd2bed 100644
--- a/default.nix
+++ b/default.nix
@@ -1,19 +1,27 @@
 {withEmacs ? false,
  nixpkgs ? (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/4aa5466cbc741097218a1c494a7b832a17d1967d.tar.gz),
- coq-version ? "8.11",
+ coq-version ? "8.12",
  print-env ? false
 }:
 with import nixpkgs {
-  # overlays = [ (super: self: {
-  #   coqPackages = { "8.11" = super.coqPackages_8_11;  }."${coq-version}".overrideScope' (self: super: {
-  #     # Coq package override example:
-  #     coq-elpi = super.coq-elpi.overrideAttrs (old: {
-  #       name = "coq8.11-elpi-1.5.0";
-  #       src = fetchTarball https://github.com/LPCIC/coq-elpi/archive/v1.5.0.tar.gz;
-  #     });
-  #   });
-  #   coq = self.coqPackages.coq;
-  # })];
+  overlays = [ (super: self: {
+    coqPackages = { "8.12" = super.coqPackages_8_12;  }."${coq-version}".overrideScope' (self: super: {
+      coq = super.coq.override {
+        ocamlPackages = super.coq.ocamlPackages.overrideScope' (self: super: {
+          elpi = super.elpi.overrideAttrs (old: {
+            name = "elpi-1.12.0";
+            src = fetchTarball https://github.com/LPCIC/elpi/archive/v1.12.0.tar.gz;
+          });
+        });
+      };
+      # Coq package override example:
+      coq-elpi = super.coq-elpi.overrideAttrs (old: {
+        name = "coq8.12-elpi-1.8.0";
+        src = fetchTarball https://github.com/LPCIC/coq-elpi/archive/v1.8.0.tar.gz;
+      });
+    });
+    coq = self.coqPackages.coq;
+  })];
 };
 let pgEmacs = emacsWithPackages (epkgs: with epkgs.melpaStablePackages; [proof-general]); in
 coqPackages.hierarchy-builder.overrideAttrs (old: {