diff --git a/darwin/configuration.nix b/darwin/configuration.nix
index 819be37..fc577ec 100644
--- a/darwin/configuration.nix
+++ b/darwin/configuration.nix
@@ -2,20 +2,28 @@
 
 {
   imports = [ ../common/nix-settings.nix ];
-  nix.settings = {
-    trusted-users = [
-      "os"
-      "@admin"
-    ]; # @admin for linux-builder
-    substituters = [
-      "https://s3.whq.fcio.net/hydra"
-      "https://hydra.flyingcircus.io"
-    ];
-    trusted-public-keys = [
-      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
-      "flyingcircus.io-1:Rr9CwiPv8cdVf3EQu633IOTb6iJKnWbVfCC8x8gVz2o="
-    ];
+  nix = {
+    enable = true;
+    package = pkgs.nixVersions.nix_2_25;
+    settings = {
+      trusted-users = [
+        "os"
+        "@admin"
+      ]; # @admin for linux-builder
+      substituters = [
+        "https://s3.whq.fcio.net/hydra"
+        "https://hydra.flyingcircus.io"
+      ];
+      trusted-public-keys = [
+        "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+        "flyingcircus.io-1:Rr9CwiPv8cdVf3EQu633IOTb6iJKnWbVfCC8x8gVz2o="
+      ];
+    };
   };
+
+  # Auto upgrade nix package and the daemon service.
+  services.nix-daemon.enable = true;
+
   # List packages installed in system profile. To search by name, run:
   # $ nix-env -qaP | grep wget
   environment.systemPackages = [ pkgs.vim ];
@@ -24,10 +32,6 @@
   # $ darwin-rebuild switch -I darwin-config=$HOME/.config/nixpkgs/darwin/configuration.nix
   # environment.darwinConfig = "$HOME/.config/nixpkgs/darwin/configuration.nix";
 
-  # Auto upgrade nix package and the daemon service.
-  services.nix-daemon.enable = true;
-  # nix.package = pkgs.nix;
-
   # Create /etc/bashrc that loads the nix-darwin environment.
   programs.zsh.enable = true; # default shell on catalina