--- x/kernel/locking/rtmutex.c
+++ y/kernel/locking/rtmutex.c
@@ -1558,6 +1558,8 @@ static void __sched remove_waiter(struct
 
 	lockdep_assert_held(&lock->wait_lock);
 
+	if (!waiter_task)
+		return;
 	scoped_guard(raw_spinlock, &waiter_task->pi_lock) {
 		rt_mutex_dequeue(lock, waiter);
 		waiter_task->pi_blocked_on = NULL;