diff --git a/packages/python/fatfs/default.nix b/packages/python/fatfs/default.nix
index db82b50..56d71bf 100644
--- a/packages/python/fatfs/default.nix
+++ b/packages/python/fatfs/default.nix
@@ -6,16 +6,23 @@
 python3Packages.buildPythonPackage rec {
   pname = "fatfs";
   version = "0.1.2";
-  pyproject = false;
+  format = "setuptools";
 
   src = python3Packages.fetchPypi {
     inherit pname version;
     hash = "sha256-gkx2ebxtJcRitGX68Xf4EAlxHP+b8wAbKpH5xb+lxYk=";
   };
 
-  build-system = with python3Packages; [
+  # wrapper.c is pre-generated in the sdist; Cython is not needed at build time.
+  # Patch out setup_requires=['cython'] to prevent setuptools from trying to
+  # fetch it via pip at build time (which fails in the Nix sandbox).
+  postPatch = ''
+    substituteInPlace setup.py \
+      --replace-fail "setup_requires=['cython']," ""
+  '';
+
+  nativeBuildInputs = with python3Packages; [
     setuptools
-    cython
   ];
 
   doCheck = false;