diff --git a/flake.nix b/flake.nix
index 538fc449..af5fd754 100644
--- a/flake.nix
+++ b/flake.nix
@@ -38,7 +38,7 @@
             };
 
             nativeBuildInputs =
-              [ python.pkgs.pdm-pep517 python.pkgs.pdm-backend ];
+              [ prev.pdm-pep517 prev.pdm-backend ];
 
             doCheck = false;
 
@@ -48,8 +48,46 @@
               maintainers = [ ];
             };
           };
+          # We need PyYAML with C support, but poetry2nix does not do that apparently...
+          # Taken from nixpkgs.
+          pyyaml = prev.buildPythonPackage rec {
+            pname = "pyyaml";
+            version = "6.0.1";
+
+            format = "pyproject";
+
+            src = pkgs.fetchFromGitHub {
+              owner = "yaml";
+              repo = "pyyaml";
+              rev = version;
+              hash = "sha256-YjWMyMVDByLsN5vEecaYjHpR1sbBey1L/khn4oH9SPA=";
+            };
+
+            nativeBuildInputs = [
+              prev.cython_0
+              prev.setuptools
+            ];
+
+            buildInputs = [ pkgs.libyaml ];
+
+            checkPhase = ''
+              runHook preCheck
+              PYTHONPATH="tests/lib:$PYTHONPATH" ${python.interpreter} -m test_all
+              runHook postCheck
+            '';
+
+            pythonImportsCheck = [ "yaml" ];
+
+            meta = with pkgs.lib; {
+              description = "The next generation YAML parser and emitter for Python";
+              homepage = "https://github.com/yaml/pyyaml";
+              license = licenses.mit;
+              maintainers = with maintainers; [ ];
+            };
+          };
         };
 
+
         # This one isn't in Nix, so do it manually.
         ast = pkgs.buildNpmPackage rec {
           pname = "abstract-syntax-tree";