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