diff --git a/nix/default.nix b/nix/default.nix
index 8f111e5..dd12bb1 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,4 +1,4 @@
 # ml2 ts=2 sts=2 sw=2
 
 with import <nixpkgs> {};
-callPackage ./package.nix {}
+{version ? "HEAD"}: callPackage ./package.nix {inherit version;}