diff --git a/artiq/pkgs/artiq-manual.nix b/artiq/pkgs/artiq-manual.nix
index ba10d83..dafb9be 100644
--- a/artiq/pkgs/artiq-manual.nix
+++ b/artiq/pkgs/artiq-manual.nix
@@ -26,7 +26,7 @@ let
 
     preBuild = ''
         export VERSIONEER_OVERRIDE=${artiqVersion}
-        export SOURCE_DATE_EPOCH=${import ./artiq-timestamp.nix {}}
+        export SOURCE_DATE_EPOCH=${import ./artiq-timestamp.nix}
         cd doc/manual
       '';
     makeFlags = [ target ];
diff --git a/artiq/pkgs/artiq-timestamp.nix b/artiq/pkgs/artiq-timestamp.nix
index cb3cf26..112b6cc 100644
--- a/artiq/pkgs/artiq-timestamp.nix
+++ b/artiq/pkgs/artiq-timestamp.nix
@@ -1,4 +1,5 @@
-{ pkgs ? import <nixpkgs> {} }:
+let pkgs = import <nixpkgs> {};
+in
 with pkgs;
 let
   artiq-timestamp = stdenv.mkDerivation {