Why is WebSocket4Net closing the connection with 1011 error code

Question

I have a Python 3 websocket server, it works fine with nodejs client wscat:

%> wscat -n -c ws://192.168.154.200:11111
connected (press CTRL+C to quit)
< Received PING
< Received PING
> your message

When I try to build another client with WebSocket4Net, it kept closing the client socket with 1011 error (every 20~30s):

In the mean while, python server with another error code 1006:

code = 1006 (connection closed abnormally [internal]), no reason

It's totally weird, I started to look for relevant code in Websocket4Net. But I never find anything useful, only a enum to Unexpected Error, and it seems to be never used.

Any ideas?