I'm pushing undef into the queue with ... and I think this will cause thread to return:

It will. Eventually. But there is no guarantee -- indeed it is most unlikely -- that all of the threads will: a) get a timeslice; b) receive the undef from the queue; c) act upon it and exit; before your main thread tries to join them.

The whole point of join is that it waits for the threads to finish, thus ensuring that your main thread doesn't exit before your child threads are finished.

It rarely makes any sense to turn it into a non-blocking operation by calling is_joinable before calling join.

The are a few specialist uses for that, but in the case of your code it only guarantees that any thread that is slow to finish will not get joined before you terminate the main thread; hence the error message.