diff --git a/default.nix b/default.nix
index eae1d56d0..97162c6e8 100644
--- a/default.nix
+++ b/default.nix
@@ -1,4 +1,6 @@
-with import <nixpkgs> { };
+{ nixpkgs ? <nixpkgs> }:
+
+with import nixpkgs { };
 
 runCommand "nix-repl"
   { buildInputs = [ readline nixUnstable boehmgc ]; }