diff --git a/pkgs/applications/networking/p2p/mldonkey/default.nix b/pkgs/applications/networking/p2p/mldonkey/default.nix
index 6c5ebdb91335..fda9d87681a9 100644
--- a/pkgs/applications/networking/p2p/mldonkey/default.nix
+++ b/pkgs/applications/networking/p2p/mldonkey/default.nix
@@ -15,6 +15,11 @@ stdenv.mkDerivation rec {
       url = "https://github.com/ygrek/mldonkey/pull/66/commits/20ff84c185396f3d759cf4ef46b9f0bd33a51060.patch";
       hash = "sha256-MCqx0jVfOaLkZhhv0b1cTdO6BK2/f6TxTWmx+NZjXME=";
     })
+    # Fixes OCaml 4.12 compat
+    (fetchpatch {
+      url = "https://github.com/ygrek/mldonkey/commit/a153f0f7a4826d86d51d4bacedc0330b70fcbc34.patch";
+      hash = "sha256-/Muk3mPFjQJ48FqaozGa7o8YSPhDLXRz9K1EyfxlzC8=";
+    })
   ];
 
   preConfigure = ''
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 0568fea44d95..07515b4087ce 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -30882,7 +30882,9 @@ with pkgs;
   mjpg-streamer = callPackage ../applications/video/mjpg-streamer { };
 
   mldonkey = callPackage ../applications/networking/p2p/mldonkey {
-    ocamlPackages = ocaml-ng.ocamlPackages_4_08;
+    ocamlPackages = ocaml-ng.mkOcamlPackages (ocaml-ng.ocamlPackages_4_13.ocaml.override {
+      unsafeStringSupport = true;
+    });
   };
 
   mlvwm = callPackage ../applications/window-managers/mlvwm { };
diff --git a/pkgs/top-level/ocaml-packages.nix b/pkgs/top-level/ocaml-packages.nix
index 8fd6b40ef19d..03bd642550f1 100644
--- a/pkgs/top-level/ocaml-packages.nix
+++ b/pkgs/top-level/ocaml-packages.nix
@@ -1640,6 +1640,7 @@ let
 
 in let inherit (pkgs) callPackage; in rec
 {
+  inherit mkOcamlPackages;
 
   ocamlPackages_4_00_1 = mkOcamlPackages (callPackage ../development/compilers/ocaml/4.00.1.nix { });