diff --git a/pkgs/applications/window-managers/dwm/default.nix b/pkgs/applications/window-managers/dwm/default.nix
index c24e4e58392..6f7a156e1aa 100644
--- a/pkgs/applications/window-managers/dwm/default.nix
+++ b/pkgs/applications/window-managers/dwm/default.nix
@@ -10,7 +10,7 @@ stdenv.mkDerivation rec {
  
   buildInputs = [ libX11 libXinerama ];
  
-  postPatch = ''sed -i "s@/usr/local@$out@" config.mk'';
+  prePatch = ''set -x; sed -i "s@/usr/local@$out@" config.mk'';
 
   # Allow users set their own list of patches
   inherit patches;