Next Websocket fast refresh doesn't work with assetPrefix

This example repository shows how the Websocket fast refresh doesn't work if assetPrefix is defined.

Running the example

• npm i
• npm run dev
• open http://localhost:3000

Notice an error in console

Uncaught DOMException: Failed to construct 'WebSocket': The URL 'ws://localhost:3000custom-asset-prefix/_next/webpack-hmr' is invalid.