diff --git a/pkgs/development/python-modules/pyhepmc/default.nix b/pkgs/development/python-modules/pyhepmc/default.nix
index 28339c457468..4e34a3bc239c 100644
--- a/pkgs/development/python-modules/pyhepmc/default.nix
+++ b/pkgs/development/python-modules/pyhepmc/default.nix
@@ -38,7 +38,7 @@ buildPythonPackage rec {
 
   dontUseCmakeConfigure = true;
 
-  CMAKE_ARGS = [ "-DEXTERNAL_PYBIND11=ON" ];
+  env.CMAKE_ARGS = toString [ "-DEXTERNAL_PYBIND11=ON" ];
 
   nativeCheckInputs = [
     graphviz