diff --git a/lass/1systems/blue/source.nix b/lass/1systems/blue/source.nix
index 8f748ab8f..a32c3a829 100644
--- a/lass/1systems/blue/source.nix
+++ b/lass/1systems/blue/source.nix
@@ -1,11 +1,14 @@
 { lib, pkgs, ... }:
 {
   nixpkgs = lib.mkForce {
-    file = toString (pkgs.fetchFromGitHub {
-      owner = "nixos";
-      repo = "nixpkgs";
-      rev = (lib.importJSON ../../../krebs/nixpkgs.json).rev;
-      sha256 = (lib.importJSON ../../../krebs/nixpkgs.json).sha256;
-    });
+    derivation = ''
+      with import <nixpkgs> {};
+      pkgs.fetchFromGitHub {
+        owner = "nixos";
+        repo = "nixpkgs";
+        rev = "${(lib.importJSON ../../../krebs/nixpkgs.json).rev}";
+        sha256 = "${(lib.importJSON ../../../krebs/nixpkgs.json).sha256}";
+      }
+    '';
   };
 }