diff --git a/Python/pystate.c b/Python/pystate.c
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -586,9 +586,9 @@
     autoInterpreterState = NULL;
 }
 
-/* Reset the TLS key - called by PyOS_AfterFork.
+/* Reset the TLS key - called by PyOS_AfterFork().
  * This should not be necessary, but some - buggy - pthread implementations
- * don't flush TLS on fork, see issue #10517.
+ * don't reset TLS upon fork(), see issue #10517.
  */
 void
 _PyGILState_Reinit(void)
@@ -598,8 +598,10 @@
     if ((autoTLSkey = PyThread_create_key()) == -1)
         Py_FatalError("Could not allocate TLS entry");
 
-    /* re-associate the current thread state with the new key */
-    if (PyThread_set_key_value(autoTLSkey, (void *)tstate) < 0)
+    /* If the thread had an associated auto thread state, reassociate it with
+     * the new key (this will not hold, for example, for a thread created
+     * outside of Python calling into a subinterpreter). */
+    if (tstate && PyThread_set_key_value(autoTLSkey, (void *)tstate) < 0)
         Py_FatalError("Couldn't create autoTLSkey mapping");
 }