diff --git a/pkgs/os-specific/windows/mcfgthreads/default.nix b/pkgs/os-specific/windows/mcfgthreads/default.nix
index 3f882557e50..122848a4e6c 100644
--- a/pkgs/os-specific/windows/mcfgthreads/default.nix
+++ b/pkgs/os-specific/windows/mcfgthreads/default.nix
@@ -19,7 +19,7 @@ stdenv.mkDerivation {
   '';
 
   postPatch = ''
-    substituteInPlace Makefile.am --replace '-Werror' ''
+    substituteInPlace Makefile.am --replace '-Werror' ""
   '';
 
   nativeBuildInputs = [