Hono Sub-App Route Handler Returns Undefined → CF Error 1101

Problem

A Cloudflare Worker using Hono with sub-apps crashes with Error 1101 ("Worker threw exception") when a route handler in a sub-app returns undefined instead of a Response. This commonly happens when route handlers have hostname guards that use bare return to skip processing, intending to let other routes handle the request.

Context / Trigger Conditions

• Cloudflare Workers dashboard or browser shows Error 1101: Worker threw exception
• wrangler dev logs show: "Incorrect type for Promise: the Promise did not resolve to 'Response'"
• Multiple hostnames route to the same worker (e.g., names.divine.video and names.admin.divine.video)
• Hono sub-app has route handlers with conditional hostname checks that use bare return
• The sub-app has middleware that calls next() for non-matching hostnames, but the route handlers duplicate the check and return undefined

Root Cause

In Hono, route handlers (.get(), .post(), etc.) MUST return a Response. Only middleware (registered via .use()) can call next() to pass control downstream.

However, if a route handler includes next in its parameter list (async (c, next) => {}), Hono treats it as middleware-like, allowing return next() to pass control to the next matching handler.

A bare return (which returns undefined) from a route handler causes the Cloudflare Workers runtime to throw because it expects a Response object.

Solution

Bad — returns undefined, crashes the worker:

app.get('/', async (c) => {
  if (!ALLOWED_HOSTNAMES.includes(hostname)) {
    return // undefined! → Error 1101
  }
  return c.html(page())
})

Good — accepts next parameter and calls it to pass control:

app.get('/', async (c, next) => {
  if (!ALLOWED_HOSTNAMES.includes(hostname)) {
    return next() // passes to next handler in chain
  }
  return c.html(page())
})

Key insight: Adding next to the handler signature changes Hono's treatment of the handler, allowing it to delegate to downstream routes.

Verification

1. Run wrangler dev and curl with the non-matching hostname:

   curl -s -o /dev/null -w "%{http_code}" http://localhost:8787/ -H "Host: other.example.com"
   

2. Should return a valid HTTP status (200, 404, etc.) instead of 500
3. No "did not resolve to Response" errors in the terminal

Example

A worker serves both names.divine.video (public UI) and names.admin.divine.video (admin SPA). The public routes sub-app has a landing page handler that only responds for names.divine.video. When names.admin.divine.video hits this handler, it should pass through to the catch-all admin SPA handler. Using bare return crashes the worker; using return next() (with next in params) properly delegates.

Notes

• This only applies to Hono route handlers, not middleware (.use() handlers always have next)
• The error is invisible in Cloudflare's dashboard — you only see generic "Error 1101"
• wrangler dev gives the real error: "the Promise did not resolve to 'Response'"
• If your sub-app middleware already guards routes, you may not need hostname checks in individual handlers at all — but if you do, use the next pattern
• This pattern is common in multi-tenant workers where one worker handles multiple hostnames