diff --git a/Lib/threading.py b/Lib/threading.py
index ab29db7..d96d99a 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -826,8 +826,12 @@
         # they may be in an invalid state leading to a deadlock or crash.
         self._started._at_fork_reinit()
         if is_alive:
-            self._tstate_lock._at_fork_reinit()
-            self._tstate_lock.acquire()
+            # bpo-42350: If the fork happens when the thread is already stopped
+            # (ex: after threading._shutdown() has been called), _tstate_lock
+            # is None. Do nothing in this case.
+            if self._tstate_lock is not None:
+                self._tstate_lock._at_fork_reinit()
+                self._tstate_lock.acquire()
         else:
             # The thread isn't alive after fork: it doesn't have a tstate
             # anymore.