How to prevent prerender error for static page that fetches data from serverless API

I'm trying to have a static props get defined by data retrieved from an endpoint on
/api/hello
from a t3 app. Running
next dev
works just fine, but when running
vercel build
, I get this error:
Error occurred prerendering page "/en". Read more: https://nextjs.org/docs/messages/prerender-error
TypeError: fetch failed
    at Object.processResponse (node:internal/deps/undici/undici:5536:34)
    at node:internal/deps/undici/undici:5858:42
    at node:internal/process/task_queues:140:7
    at AsyncResource.runInAsyncScope (node:async_hooks:202:9)
    at AsyncResource.runMicrotask (node:internal/process/task_queues:137:8)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
info  - Generating static pages (5/5)

> Build error occurred
Error: Export encountered errors on following paths:
        /: /en
    at /Users/alann/code/next-ssg-edge/node_modules/.pnpm/next@12.3.1_biqbaboplfbrettd7655fr4n2y/node_modules/next/dist/export/index.js:404:19
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Span.traceAsyncFn 
    at async /Users/alann/code/next-ssg-edge/node_modules/.pnpm/next@12.3.1_biqbaboplfbrettd7655fr4n2y/node_modules/next/dist/build/index.js:1091:17
    at async Span.traceAsyncFn 

(cut to fit character limit)
I see that
fetch failed
so it might just be that the env var isn't defined at the time, since I'm fetching from
await fetch(http://${process.env.NEXT_PUBLIC_VERCEL_URL}/api/graphql)
. The thing is I already have
.env.development.local
defined and I see this msg:
info  - Loaded env from /Users/alann/code/next-ssg-edge/.env.local
. Thank you.
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