Can't upgrade request to websocket

[uncaught application error]: TypeError - Invalid Header: 'upgrade' header must contain 'websocket'

import {Application,Router} from 'https://deno.land/x/oak@v12.6.1/mod.ts'

function RefreshMiddleware() {
  return async (ctx,next) => {
    console.log('url',ctx.request.url.href)
    if (ctx.request.url.href.endsWith("/refresh")) {
      console.log(ctx.isUpgradable);
      console.log(ctx.request.headers.get("connection"));
      const socket = ctx.upgrade()
    } else next()
  }
}

const app = new Application()
app.addEventListener("error", e => console.error(e.error))

// Logger
app.use(async (ctx, next) => {
  const start = Date.now()
  await next()
  console.log(`${ctx.request.method} ${ctx.request.url} - ${Date.now() - start}ms`)
})
app.use(RefreshMiddleware())
app.use((ctx) => {ctx.response.body = "Hello World!"})

await app.listen({port: 8000})