NWConnection Websocket send error

I'm attempting to use NWConnection as a websocket given a NWEndpoint returned by NWBrowser, setup like:

    let tcpOptions = NWProtocolTCP.Options()
    tcpOptions.enableKeepalive = true
    tcpOptions.keepaliveIdle = 2
    let parameters = NWParameters(tls: nil, tcp: tcpOptions)
    parameters.allowLocalEndpointReuse = true
    parameters.includePeerToPeer = true
    let options = NWProtocolWebSocket.Options()
    options.autoReplyPing = true
    options.skipHandshake = true

    parameters.defaultProtocolStack.applicationProtocols.insert(options, at: 0)
    self.connection = NWConnection(to: endpoint, using: parameters)

The initial connection does make it to the ready state but when I first try to send a text message over the websocket, i get

nw_read_request_report [C1] Receive failed with error "Input/output error"
nw_flow_prepare_output_frames Failing the write requests [5: Input/output error]
nw_write_request_report [C1] Send failed with error "Input/output error"

immediately, and the websocket is closed. Send code here:

      let encoder = JSONEncoder()
      let dataMessage = try encoder.encode(myMessage)
      let messageMetadata = NWProtocolWebSocket.Metadata(opcode: .text)
      let context = NWConnection.ContentContext(identifier: "send", metadata: [messageMetadata])
      connection.send(content: dataMessage, contentContext: context, completion: .contentProcessed({ error in
        if let error = error {
          print (error)
        }
      }))

What would typically cause the Input/output error when writing? Am I doing something obviously wrong or is there something else I can do to get additional debug information?

Thanks in advance for any help.