diff --git a/pkgs/development/python-modules/tokenizers/default.nix b/pkgs/development/python-modules/tokenizers/default.nix
index 544dbf1beafc..7813797a2f31 100644
--- a/pkgs/development/python-modules/tokenizers/default.nix
+++ b/pkgs/development/python-modules/tokenizers/default.nix
@@ -49,19 +49,19 @@ let
   };
 in buildPythonPackage rec {
   pname = "tokenizers";
-  version = "0.10.3";
+  version = "unstable-2021-08-13";
 
   src = fetchFromGitHub {
     owner = "huggingface";
     repo = pname;
-    rev = "python-v${version}";
-    hash = "sha256-X7aUiJJjB2ZDlE8LbK7Pn/15SLTZbP8kb4l9ED7/xvU=";
+    rev = "e7dd6436dd4a4ffd9e8a4f110ca68e6a38677cb6";
+    sha256 = "1p7w9a43a9h6ys5nsa4g89l65dj11037p7a1lqkj4x1yc9kv2y1r";
   };
 
   cargoDeps = rustPlatform.fetchCargoTarball {
     inherit src sourceRoot;
     name = "${pname}-${version}";
-    hash = "sha256-gRqxlL6q87sGC0birDhCmGF+CVbfxwOxW6Tl6+5mGoo=";
+    sha256 = "1yb4jsx6mp9jgd1g3mli6vr6mri2afnwqlmxq1rpvn34z6b3iw9q";
   };
 
   sourceRoot = "source/bindings/python";
@@ -97,6 +97,10 @@ in buildPythonPackage rec {
       ln -s ${openaiMerges} openai-gpt-merges.txt )
   '';
 
+  postPatch = ''
+    echo 'import multiprocessing; multiprocessing.set_start_method("fork")' >> tests/__init__.py
+  '';
+
   preCheck = ''
     HOME=$TMPDIR
   '';