diff --git a/pkgs/development/interpreters/python/hooks/setuptools-build-hook.sh b/pkgs/development/interpreters/python/hooks/setuptools-build-hook.sh
index 89a95e982197..958a9378ef14 100644
--- a/pkgs/development/interpreters/python/hooks/setuptools-build-hook.sh
+++ b/pkgs/development/interpreters/python/hooks/setuptools-build-hook.sh
@@ -12,10 +12,10 @@ setuptoolsBuildPhase() {
         args+="$setupPyGlobalFlags"
     fi
     if [ -n "$enableParallelBuilding" ]; then
-        setupPyBuildFlags+="--parallel $NIX_BUILD_CORES"
+        setupPyBuildFlags+=" --parallel $NIX_BUILD_CORES"
     fi
     if [ -n "$setupPyBuildFlags" ]; then
-        args+="build_ext $setupPyBuildFlags"
+        args+=" build_ext $setupPyBuildFlags"
     fi
     eval "@pythonInterpreter@ nix_run_setup $args bdist_wheel"