diff --git a/pkgs/development/libraries/nss/default.nix b/pkgs/development/libraries/nss/default.nix
index e559b9a076ac..4e6911058de3 100644
--- a/pkgs/development/libraries/nss/default.nix
+++ b/pkgs/development/libraries/nss/default.nix
@@ -5,13 +5,15 @@ let
     url = http://dev.gentoo.org/~polynomial-c/mozilla/nss-3.15.4-pem-support-20140109.patch.xz;
     sha256 = "10ibz6y0hknac15zr6dw4gv9nb5r5z9ym6gq18j3xqx7v7n3vpdw";
   };
+  version = "3.40";
+  underscoreVersion = builtins.replaceStrings ["."] ["_"] version;
 
 in stdenv.mkDerivation rec {
   name = "nss-${version}";
-  version = "3.40";
+  inherit version;
 
   src = fetchurl {
-    url = "mirror://mozilla/security/nss/releases/NSS_3_40_RTM/src/${name}.tar.gz";
+    url = "mirror://mozilla/security/nss/releases/NSS_${underscoreVersion}_RTM/src/${name}.tar.gz";
     sha256 = "1p9jidi3nysirf3lhkrqkjjrf2cw3y2wz2jgjvsjn1ysidxhhqh5";
   };