diff --git a/pkgs/tools/misc/units/default.nix b/pkgs/tools/misc/units/default.nix
index 04e955a78b189..ebfd913de217d 100644
--- a/pkgs/tools/misc/units/default.nix
+++ b/pkgs/tools/misc/units/default.nix
@@ -9,7 +9,10 @@
 
 assert enableCurrenciesUpdater -> pythonPackages != null;
 
-stdenv.mkDerivation rec {
+let pythonEnv = pythonPackages.python.withPackages(ps: [
+      ps.requests
+    ]);
+in stdenv.mkDerivation rec {
   pname = "units";
   version = "2.21";
 
@@ -18,16 +21,12 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-bD6AqfmAWJ/ZYqWFKiZ0ZCJX2xxf1bJ8TZ5mTzSGy68=";
   };
 
-  pythonEnv = pythonPackages.python.withPackages(ps: [
-    ps.requests
-  ]);
-
   buildInputs = [ readline ]
     ++ lib.optionals enableCurrenciesUpdater [
       pythonEnv
     ]
   ;
-  prePatch = ''
+  prePatch = lib.optionalString enableCurrenciesUpdater ''
     substituteInPlace units_cur \
       --replace "#!/usr/bin/env python" ${pythonEnv}/bin/python
   '';