diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index 151f8a30..bca658c0 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -184,7 +184,7 @@ def pep_518_cp35_workaround(package_dir: Path, env: Dict[str, str]) -> None:
                 with reqfile.open("w") as f:
                     for r in requirements:
                         print(r, file=f)
-                call(['pip', 'install', '-r', str(reqfile)], env=env)
+                call(['pip', 'install', '-r', reqfile], env=env)
 
 
 def build(options: BuildOptions) -> None:
diff --git a/test/test_pep518.py b/test/test_pep518.py
index 58ed2c71..77482f93 100644
--- a/test/test_pep518.py
+++ b/test/test_pep518.py
@@ -50,6 +50,8 @@ def test_pep518(tmp_path):
     expected_wheels = utils.expected_wheels("spam", "0.1.0")
     assert set(actual_wheels) == set(expected_wheels)
 
+    # These checks ensure an extra file is not created when using custom
+    # workaround; see https://github.com/joerick/cibuildwheel/issues/421
     assert not (project_dir / "42").exists()
     assert not (project_dir / "4.1.2").exists()