Got "Invalid handshake" if I turn on embedded mqtt

I got this Invalid handshake warning every two minutes if I turned on the embedded MQTT.

Invalid handshake
2:11 AM deps/lib/python3.5/site-packages/websockets/server.py (WARNING)

But the mqtt seems to be working cuz my sonoff switch can work fine.
Any idea what’s wrong?
Thanks!

37%20AM