ws: TypeError: Cannot read property 'unmask' of undefined at Receiver.unmask

I have a very simple node app that uses socket.io. When I connect to the server from an external client it works fine. However, when I attempt to render UI using a browser, the server crashes on TypeError from Receiver.unmask.

My server code:

const express = require('express');
const app = express();
const http = require('http').createServer(app);
const io = require('socket.io')(http);

const PORT = process.env.PORT || 8080;

app.use(express.static('public/dashboard'));

app.get('/', (req, res) => {
  res.sendFile(__dirname + '/public/dashboard/index.html');
});

io.on('connection', (client) => {
  console.log('Client connected');
  client.on('disconnect', () => console.log('Client disconnected'));
});

// broadcast PING to all connected sockets
// hoping to hear back from bots
setInterval(() => io.emit('PING', new Date().toTimeString()), 5000);

http.listen(PORT, function(){
  console.log(`listening on *:${PORT}`);
});

UI code:

bunch of HTML and JS
    <script src="/socket.io/socket.io.js"></script>
    <script>
      var socket = io();
    </script>

Error on node server side:

  socket.io:socket socket connected - writing packet +4ms
  socket.io:socket joining room Ipoywx491U9SrqRxAAAA +1ms
  socket.io:client writing packet {"type":0,"nsp":"/"} +1ms
  socket.io-parser encoding packet {"type":0,"nsp":"/"} +2s
  socket.io-parser encoded {"type":0,"nsp":"/"} as 0 +0ms
  engine:socket sending packet "message" (0) +10ms
  socket.io:socket joined room Ipoywx491U9SrqRxAAAA +4ms
  engine upgrading existing transport +16ms
  engine:socket might upgrade socket transport from "polling" to "websocket" +1ms
[my_workspace]/node_modules/ws/lib/Receiver.js:306
  if (mask != null && buf != null) bufferUtil.unmask(buf, mask);
                                             ^
TypeError: Cannot read property 'unmask' of undefined
    at Receiver.unmask ([my_workspace]/node_modules/ws/lib/Receiver.js:306:46)
    at Receiver.finish ([my_workspace]/node_modules/ws/lib/Receiver.js:505:25)
    at Receiver.expectHandler ([my_workspace]/node_modules/ws/lib/Receiver.js:493:33)
    at Receiver.add ([my_workspace]/node_modules/ws/lib/Receiver.js:103:24)
    at Socket.realHandler ([my_workspace]/node_modules/ws/lib/WebSocket.js:825:20)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at readableAddChunk (_stream_readable.js:176:18)
    at Socket.Readable.push (_stream_readable.js:134:10)
    at TCP.onread (net.js:548:20)

Error in UI:

engine.io-client:socket probe transport "websocket" opened +132ms
universalModuleDefinition:1 engine.io-client:polling polling got data ArrayBuffer +3ms
universalModuleDefinition:1 engine.io-client:socket socket receive: type "message", data "0" +1ms
universalModuleDefinition:1 socket.io-parser decoded 0 as {"type":0,"nsp":"/"} +0ms
universalModuleDefinition:1 engine.io-client:polling polling +5ms
universalModuleDefinition:1 engine.io-client:polling-xhr xhr poll +0ms
universalModuleDefinition:1 engine.io-client:polling-xhr xhr open GET: http://localhost:8080/socket.io/?EIO=3&transport=polling&t=LfNIZqL&sid=LZfZtocuqb1C9V0fAAAA +1ms
universalModuleDefinition:1 engine.io-client:polling-xhr xhr data null +1ms
universalModuleDefinition:1 engine.io-client:socket probe transport "websocket" failed because of error: transport closed +20ms
VM445:2 GET http://localhost:8080/socket.io/?EIO=3&transport=polling&t=LfNIZqL&sid=LZfZtocuqb1C9V0fAAAA net::ERR_CONNECTION_REFUSED
(anonymous) @ VM445:2
i.create @ universalModuleDefinition:2
i @ universalModuleDefinition:2
o.request @ universalModuleDefinition:2
o.doPoll @ universalModuleDefinition:2
n.poll @ universalModuleDefinition:2
n.onData @ universalModuleDefinition:2
(anonymous) @ universalModuleDefinition:2
n.emit @ universalModuleDefinition:2
i.onData @ universalModuleDefinition:2
i.onLoad @ universalModuleDefinition:2
hasXDR.r.onreadystatechange @ universalModuleDefinition:2
universalModuleDefinition:1 engine.io-client:socket socket error {"type":"TransportError","description":0} +13ms
universalModuleDefinition:1 socket.io-client:manager error +5ms Error: xhr poll error
    at o.n.onError (universalModuleDefinition:2)
    at i.<anonymous> (universalModuleDefinition:2)
    at i.n.emit (universalModuleDefinition:2)
    at i.onError (universalModuleDefinition:2)
    at universalModuleDefinition:2
universalModuleDefinition:1 engine.io-client:socket socket close with reason: "transport error" +8ms

Using Node v6.8.1, socket.io 1.7.3 that uses ws@1.1.2. Is this a known issue or am I missing something obvious?