koa-router behaves different in Deno

import Koa from 'koa/lib/application';
import KoaRouter from '@koa/router';

const app = new Koa({});
const router = new KoaRouter();

app.use(router.routes())
router.get("/", (ctx, next) => {
  ctx.type = "text/javascript"
  ctx.body = "console.log('hello')"
})

app.listen(2000)