according to https://github.com/netty/netty/blob/0514b0c61be19119b420fa661123990acbdd8305/codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocketClientHandshaker.java#L228, a receivedProtocol needs to be supplied to the WebSocketClientHandshaker upon construction. It has to match one of the values set in the header. I’m not sure how the WSCHandshaker is getting created though, but the exception indicates that the receivedProtocol is set to null.

seems you need to pass a sub-protocols comma-delimited protocol string to the websocket-client in Aleph - https://github.com/ztellman/aleph/blob/2ba3484ead3a8443667d3a3caddf9739c6841d9c/src/aleph/http/client.clj#L379

@kurt-o-sys ^

@dm3 Thx... I may check it some day. For now, I'm just using a java lib (`jawampa`)... it implements already the protocol pretty well, so no need to use something else right now (it's only to use in a client, and that ws is about the only thing to setup to fetch data)