asyncio.selector_events.BaseSelectorEventLoop._sock_connect_cb is a callback based on the selector for a socket. There are certain situations when the selector triggers twice calling this callback twice, resulting in an InvalidStateError when it sets the Future to None. The way I triggered this was by having several parallel connections to the same host in a multiprocessing script. I suggest analyzing why this callback can be called twice and figuring out what the correct fix is. I monkey patched it by adding a fut.done() check at the top. If this information is not enough I can try to provide a sample script. Its currently reproducing in a fairly involved multiprocessing script.