diff --git a/distutils/_msvccompiler.py b/distutils/_msvccompiler.py
index b7a06082..c41ea9ae 100644
--- a/distutils/_msvccompiler.py
+++ b/distutils/_msvccompiler.py
@@ -527,7 +527,7 @@ def _fallback_spawn(self, cmd, env):
             return
         warnings.warn(
             "Fallback spawn triggered. Please update distutils monkeypatch.")
-        with unittest.mock.patch('os.environ', env):
+        with unittest.mock.patch.dict('os.environ', env):
             bag.value = super().spawn(cmd)
 
     # -- Miscellaneous methods -----------------------------------------