diff --git a/flake.lock b/flake.lock
index c2ce607..2d85726 100644
--- a/flake.lock
+++ b/flake.lock
@@ -178,6 +178,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1655415671,
+        "narHash": "sha256-WD7HxxW1m8D/fkV1QlCYlZvnE5gQdg7ckq3myI4gPtE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "f96729212602f15a6a226d2f27f5de70492ad095",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-21.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1655409759,
@@ -238,6 +254,7 @@
         "nasin-nanpa": "nasin-nanpa",
         "nixpkgs": "nixpkgs_2",
         "nixpkgs-go116": "nixpkgs-go116",
+        "nixpkgs-stable": "nixpkgs-stable",
         "plover": "plover",
         "regenpfeifer": "regenpfeifer",
         "wortformliste": "wortformliste"
diff --git a/flake.nix b/flake.nix
index c03a3e3..b802835 100644
--- a/flake.nix
+++ b/flake.nix
@@ -3,6 +3,7 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs";
+    nixpkgs-stable.url = "github:NixOS/nixpkgs/nixos-21.11";
     flake-utils.url = "github:numtide/flake-utils";
 
     # Source files for packages
diff --git a/misc/plover.nix b/misc/plover.nix
index c2039a3..e185507 100644
--- a/misc/plover.nix
+++ b/misc/plover.nix
@@ -2,7 +2,9 @@
   inputs,
   pkgs,
 }:
-with pkgs; rec {
+with pkgs; let
+  inherit (inputs.nixpkgs-stable.legacyPackages.${system}) pypy3;
+in rec {
   plover-plugins-manager = with python3Packages;
     buildPythonPackage rec {
       pname = "plover-plugins-manager";
@@ -53,7 +55,7 @@ with pkgs; rec {
       };
       propagatedBuildInputs = [plover];
     };
-    regenpfeifer-env = python3.withPackages(ps: [ps.marisa-trie]);
+  regenpfeifer-env = pypy3.withPackages (ps: [ps.marisa-trie]);
   wortformliste = pkgs.stdenvNoCC.mkDerivation {
     pname = "wortformliste";
     version = inputs.wortformliste.lastModifiedDate;
@@ -68,7 +70,7 @@ with pkgs; rec {
     src = inputs.regenpfeifer;
     nativeBuildInputs = [regenpfeifer-env];
     buildPhase = ''
-      python3 -m regenpfeifer.dictionary_generator ${wortformliste} $out unmatched.log 300000 300000
+      pypy3 -m regenpfeifer.dictionary_generator ${wortformliste} $out unmatched.log 300000 300000
     '';
     installPhase = "cat unmatched.log";
   };