diff --git a/pkgs/development/python-modules/cwlformat/default.nix b/pkgs/development/python-modules/cwlformat/default.nix
index 28da8e50beeb..0703175d0275 100644
--- a/pkgs/development/python-modules/cwlformat/default.nix
+++ b/pkgs/development/python-modules/cwlformat/default.nix
@@ -1,6 +1,7 @@
 { lib
 , buildPythonPackage
 , fetchFromGitHub
+, fetchpatch
 , pytestCheckHook
 , pythonOlder
 , ruamel-yaml
@@ -20,6 +21,15 @@ buildPythonPackage rec {
     hash = "sha256-FI8hUgb/KglTkubZ+StzptoSsYal71ITyyFNg7j48yk=";
   };
 
+  patches = [
+    # https://github.com/rabix/cwl-format/pull/21
+    (fetchpatch {
+      name = "fix-for-ruamel-yaml-0.17.23.patch";
+      url = "https://github.com/rabix/cwl-format/commit/9d54330c73c454d2ccacd55e2d51a4145f282041.patch";
+      hash = "sha256-TZGK7T2gzxMvreCLtl3nkuPrqL2KzgrO3yCNmd5lY3g=";
+    })
+  ];
+
   propagatedBuildInputs = [
     ruamel-yaml
   ];