diff --git a/pkgs/applications/version-management/monotone/default.nix b/pkgs/applications/version-management/monotone/default.nix
index fbd36417bfc..70e335ea649 100644
--- a/pkgs/applications/version-management/monotone/default.nix
+++ b/pkgs/applications/version-management/monotone/default.nix
@@ -1,22 +1,30 @@
-{stdenv, fetchurl, boost, zlib, botan, libidn,
-  lua, pcre, sqlite, perl, pkgconfig}:
+{ stdenv, fetchurl, boost, zlib, botan, libidn
+, lua, pcre, sqlite, perl, pkgconfig }:
 
 let 
   version = "0.99.1";
-  perlVersion = builtins.substring 5 10 perl.name;
-in stdenv.mkDerivation rec {
+  perlVersion = (builtins.parseDrvName perl.name).version;
+in
+
+assert perlVersion != "";
+
+stdenv.mkDerivation rec {
   name = "monotone-${version}";
+  
   src = fetchurl {
     url = "http://monotone.ca/downloads/${version}/monotone-${version}.tar.gz";
     sha256 = "189h5f6gqd4ng0qmzi3xwnj17nnpxm2vzras216ar6b5yc9bnki0";
   };
+  
   buildInputs = [boost zlib botan libidn lua pcre sqlite pkgconfig];
+  
   postInstall = ''
     ensureDir $out/share/${name}
     cp -rv contrib/ $out/share/${name}/contrib
     ensureDir $out/lib/perl5/site_perl/${perlVersion}
     cp -v contrib/Monotone.pm $out/lib/perl5/site_perl/${perlVersion}
   '';
+  
   meta = {
     description = "A free distributed version control system";
     maintainers = [stdenv.lib.maintainers.raskin];