hono-subapp-undefined-response-1101

Testing & Quality

Fix Cloudflare Workers Error 1101 "Worker threw exception" caused by Hono sub-app route handlers returning undefined instead of a Response. Use when: (1) Error 1101 on a specific hostname but not others sharing the same worker, (2) "the Promise did not resolve to 'Response'" in wrangler dev logs, (3) Hono sub-app route handler uses bare `return` to skip handling for non-matching hostnames or conditions. The fix is to add `next` parameter and call `return next()` instead of bare `return` in route handlers that conditionally skip.

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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

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"

Should return a valid HTTP status (200, 404, etc.) instead of 500
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