diff --git a/pkgs/development/python-modules/nbmake/default.nix b/pkgs/development/python-modules/nbmake/default.nix
index d070f39853cb..46ffe613f695 100644
--- a/pkgs/development/python-modules/nbmake/default.nix
+++ b/pkgs/development/python-modules/nbmake/default.nix
@@ -50,6 +50,10 @@ buildPythonPackage rec {
 
   pythonImportsCheck = [ "nbmake" ];
 
+  # tests are prone to race conditions under high parallelism
+  # https://github.com/treebeardtech/nbmake/issues/129
+  pytestFlagsArray = [ "--maxprocesses=4" ];
+
   nativeCheckInputs = [
     pytest-xdist
     pytestCheckHook