diff --git a/testing/test_runner.py b/testing/test_runner.py
index d3ddc5d9a..ffac0f2d9 100644
--- a/testing/test_runner.py
+++ b/testing/test_runner.py
@@ -1097,7 +1097,7 @@ def test_outcome_exception_bad_msg() -> None:
 
 
 def test_pytest_version_env_var(pytester: Pytester, monkeypatch: MonkeyPatch) -> None:
-    os.environ["PYTEST_VERSION"] = "old version"
+    monkeypatch.setenv("PYTEST_VERSION", "old version")
     pytester.makepyfile(
         """
         import pytest