ConnectionError, Max retries exceeded with url, (Caused by None)

What does it mean when it says "requests.exceptions.ConnectionError: None: Max retries exceeded with url: /myurl (Caused by None)"?

Specifically what does "Caused by None" mean?

I have a python client and a simple clojure server running on the same machine. The server runs on compojure+http-kit with 4 threads. The client continuously submit GET requests using 3 to 4 processes using multiprocessing.Pool with 3 worker processes.

Every so often, the client would die with the ConnectionError described above. I have settings retries=3 on the client side and increasing the queue size on the server to 1000000 with no effect.

Any help would be appreciated.