diff --git a/tests/conftest.py b/tests/conftest.py
index 0978b2ef..438ced13 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -57,10 +57,10 @@ def isolated_pio_core(request, tmpdir_factory):
 
     def fin():
         for key, item in backup_env_vars.items():
-            if item["old"] is None:
-                del os.environ[key]
-            else:
+            if item["old"] is not None:
                 os.environ[key] = item["old"]
+            elif key in os.environ:
+                del os.environ[key]
 
     request.addfinalizer(fin)
     return core_dir