diff --git a/Modules/_multiprocessing/semaphore.c b/Modules/_multiprocessing/semaphore.c
--- a/Modules/_multiprocessing/semaphore.c
+++ b/Modules/_multiprocessing/semaphore.c
@@ -108,16 +108,18 @@ semlock_acquire(SemLockObject *self, PyO
 
     /* prepare list of handles */
     nhandles = 0;
     handles[nhandles++] = self->handle;
     if (_PyOS_IsMainThread()) {
         sigint_event = _PyOS_SigintEvent();
         assert(sigint_event != NULL);
         handles[nhandles++] = sigint_event;
+    } else {
+        sigint_event = NULL;
     }
 
     /* do the wait */
     Py_BEGIN_ALLOW_THREADS
     if (sigint_event != NULL)
         ResetEvent(sigint_event);
     res = WaitForMultipleObjectsEx(nhandles, handles, FALSE, full_msecs, FALSE);
     Py_END_ALLOW_THREADS