How to connect to websocket in 1.7

Traefik Traefik v1
5

The response header received by firefox:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: RoEPtLCdXyctgN+TUFI0sfC+R1E=
Sec-WebSocket-Protocol: graphql-ws

And my gql playground displays:

{
  "error": {
    "message": "Expected {...} to be a GraphQL schema."
  }
}

The protocol is being switched. could it be the [entryPoints.http.redirect] i specified in traefik.toml?