diff --git a/pkgs/development/ocaml-modules/camlimages/default.nix b/pkgs/development/ocaml-modules/camlimages/default.nix
index 0d08cc48cb7b..b1d3d51c8666 100644
--- a/pkgs/development/ocaml-modules/camlimages/default.nix
+++ b/pkgs/development/ocaml-modules/camlimages/default.nix
@@ -6,8 +6,6 @@ buildDunePackage rec {
   pname = "camlimages";
   version = "5.0.4";
 
-  duneVersion = "3";
-
   minimalOCamlVersion = "4.07";
 
   src = fetchFromGitLab {
@@ -17,6 +15,10 @@ buildDunePackage rec {
     sha256 = "1m2c76ghisg73dikz2ifdkrbkgiwa0hcmp21f2fm2rkbf02rq3f4";
   };
 
+  postPatch = ''
+    substituteInPlace core/{images,units}.ml --replace String.lowercase String.lowercase_ascii
+  '';
+
   nativeBuildInputs = [ cppo ];
   buildInputs = [ dune-configurator findlib graphics lablgtk stdio ];