diff --git a/pkgs/by-name/cm/cmake/setup-hook.sh b/pkgs/by-name/cm/cmake/setup-hook.sh
index 3df8b9bd986b..a6f5a4f57a5b 100755
--- a/pkgs/by-name/cm/cmake/setup-hook.sh
+++ b/pkgs/by-name/cm/cmake/setup-hook.sh
@@ -131,6 +131,9 @@ cmakeConfigurePhase() {
         enableParallelBuilding=1
         echo "cmake: enabled parallel building"
     fi
+    if [[ "$enableParallelBuilding" -ne 0 ]]; then
+        export CMAKE_BUILD_PARALLEL_LEVEL=$NIX_BUILD_CORES
+    fi
 
     if ! [[ -v enableParallelInstalling ]]; then
         enableParallelInstalling=1
diff --git a/pkgs/development/python-modules/pyhepmc/default.nix b/pkgs/development/python-modules/pyhepmc/default.nix
index 427d9ecabeba..44ee5b78be85 100644
--- a/pkgs/development/python-modules/pyhepmc/default.nix
+++ b/pkgs/development/python-modules/pyhepmc/default.nix
@@ -43,10 +43,6 @@ buildPythonPackage rec {
 
   CMAKE_ARGS = [ "-DEXTERNAL_PYBIND11=ON" ];
 
-  preBuild = ''
-    export CMAKE_BUILD_PARALLEL_LEVEL="$NIX_BUILD_CORES"
-  '';
-
   nativeCheckInputs = [
     graphviz
     pytestCheckHook