diff --git a/nix/default.nix b/nix/default.nix
new file mode 100644
index 0000000..8f111e5
--- /dev/null
+++ b/nix/default.nix
@@ -0,0 +1,4 @@
+# ml2 ts=2 sts=2 sw=2
+
+with import <nixpkgs> {};
+callPackage ./package.nix {}
diff --git a/nix/package.nix b/nix/package.nix
index 14f6846..4404d39 100644
--- a/nix/package.nix
+++ b/nix/package.nix
@@ -1,7 +1,8 @@
 # ml2 ts=2 sts=2 sw=2
-{buildGoModule, version}: buildGoModule {
+{buildGoModule, version ? "HEAD"}: buildGoModule {
   pname = "tt";
   inherit version;
+
   # In 'nix develop', we don't need a copy of the source tree
   # in the Nix store.
   src = ./..;