diff --git a/pkgs/by-name/fb/fb303/package.nix b/pkgs/by-name/fb/fb303/package.nix
index 18e0ea7c784c..4d3ab9a124a5 100644
--- a/pkgs/by-name/fb/fb303/package.nix
+++ b/pkgs/by-name/fb/fb303/package.nix
@@ -44,7 +44,7 @@ stdenv.mkDerivation (finalAttrs: {
     ];
 
   cmakeFlags = [
-    "-DPYTHON_EXTENSIONS=OFF"
+    (lib.cmakeBool "PYTHON_EXTENSIONS" false)
   ];
 
   meta = {