diff --git a/setuptools/tests/test_bootstrap.py b/setuptools/tests/test_bootstrap.py
index 58c836ffe70..8e7c5f88a7b 100644
--- a/setuptools/tests/test_bootstrap.py
+++ b/setuptools/tests/test_bootstrap.py
@@ -44,5 +44,8 @@ def test_bootstrap_sourcetree(tmp_path, bare_venv, setuptools_sourcetree):
     assert not (target / "setuptools/tests").is_dir()
     assert not (target / "pkg_resources/tests").is_dir()
 
+    # Avoid errors on Windows by copying env before modifying
+    # https://stackoverflow.com/questions/58997105
+    env = os.environ.copy().update(PYTHONPATH=str(target))
     test = ["python", "-c", "print(__import__('setuptools').__version__)"]
-    bare_venv.run(test, env={"PYTHONPATH": str(target)})
+    bare_venv.run(test, env=env)