http-proxy-middleware: TypeError: Cannot read property 'on' of null at catchUpgradeRequest

Expected behavior

Able to proxy websockets with webpack 2.

Actual behavior

Not able to proxy websockets with webpack 2.

Setup

webpack-dev-server v2.2.1 was just pushed to stable. It depends on http-proxy-middleware ~0.17.1, which resolves to 0.17.3.

At the previous stable version, v1.16.2, this webpack config works:

  devServer: {
    https: true,
    proxy: {
      '/my-backend': {
        changeOrigin: true,
        pathRewrite: {'^/my-backend': ''},
        target: myBackendUrl,
        ws: true,
      },
  }

At v2.2.1, it does not. Instead I get:

TypeError: Cannot read property 'on' of null
    at catchUpgradeRequest (/work/my-project/node_modules/http-proxy-middleware/lib/index.js:56:19)
    at middleware (/work/my-project/node_modules/http-proxy-middleware/lib/index.js:48:13)
    at /work/my-project/node_modules/webpack-dev-server/lib/Server.js:223:15
    at Layer.handle [as handle_request] (/work/my-project/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/work/my-project/node_modules/express/lib/router/index.js:312:13)
    at /work/my-project/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/work/my-project/node_modules/express/lib/router/index.js:330:12)
    at next (/work/my-project/node_modules/express/lib/router/index.js:271:10)
    at middleware (/work/my-project/node_modules/http-proxy-middleware/lib/index.js:43:13)
    at /work/my-project/node_modules/webpack-dev-server/lib/Server.js:223:15
    at Layer.handle [as handle_request] (/work/my-project/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/work/my-project/node_modules/express/lib/router/index.js:312:13)
    at /work/my-project/node_modules/express/lib/router/index.js:280:7
    at Function.process_params (/work/my-project/node_modules/express/lib/router/index.js:330:12)
    at next (/work/my-project/node_modules/express/lib/router/index.js:271:10)
    at goNext (/work/my-project/node_modules/webpack-dev-middleware/middleware.js:28:49)