diff --git a/default.nix b/default.nix
index 26c4434..0e423cf 100644
--- a/default.nix
+++ b/default.nix
@@ -2,6 +2,7 @@
   pkgs ? import ./pkgs.nix,
 }: let
   lib = pkgs.lib;
+  stdenv = pkgs.stdenv;
   rustPlatform = pkgs.rustPlatform;
 in rustPlatform.buildRustPackage {
   name = "colmena-dev";
@@ -16,14 +17,12 @@ in rustPlatform.buildRustPackage {
   };
   cargoSha256 = "10h8bsy8hh36mvdgpnbw4vxnzkxyfw2vb4r1pn7fxfn0nklgakq7";
 
-  postBuild = ''
+  postInstall = lib.optionalString (stdenv.hostPlatform == stdenv.buildPlatform) ''
     mkdir completions
     for shell in bash fish zsh; do
-      cargo run --frozen -- gen-completions $shell > completions/$shell
+      $out/bin/colmena gen-completions $shell > completions/$shell
     done
-  '';
 
-  postInstall = ''
     mkdir -p "$out/share/"{bash-completion/completions,fish/vendor_completions.d,zsh/site-functions}
     cp completions/bash $out/share/bash-completion/completions/colmena
     cp completions/fish $out/share/fish/vendor_completions.d/colmena.fish