diff --git a/pkgs/tools/security/signify/default.nix b/pkgs/tools/security/signify/default.nix
index fb3df51167b..3ac6f58dce9 100644
--- a/pkgs/tools/security/signify/default.nix
+++ b/pkgs/tools/security/signify/default.nix
@@ -16,10 +16,12 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ libbsd ];
 
-  preInstall = ''
-    export PREFIX=$out
+  postPatch = ''
+    substituteInPlace Makefile --replace "shell pkg-config" "shell $PKG_CONFIG"
   '';
 
+  installFlags = [ "PREFIX=$(out)" ];
+
   meta = with lib; {
     description = "OpenBSD signing tool";
     longDescription = ''