How do I debug "Error: Node streams are no longer supported — use a ReadableStream instead"

[alexbjorlig]

I recently upgraded "@sveltejs/kit": "^1.0.0-next.357", and "@sveltejs/adapter-node": "^1.0.0-next.78",.

Now we start to see the following

Error: Node streams are no longer supported — use a ReadableStream instead
2022-07-05T19:56:35.585362+00:00 app[web.1]:     at is_pojo (file:///app/apps/web-app/build/server/index.js:102:13)
2022-07-05T19:56:35.585363+00:00 app[web.1]:     at render_endpoint (file:///app/apps/web-app/build/server/index.js:172:7)
2022-07-05T19:56:35.585363+00:00 app[web.1]:     at processTicksAndRejections (node:internal/process/task_queues:96:5)
2022-07-05T19:56:35.585363+00:00 app[web.1]:     at async resolve (file:///app/apps/web-app/build/server/index.js:2155:53)
2022-07-05T19:56:35.585364+00:00 app[web.1]:     at async Object.handle (file:///app/apps/web-app/build/server/chunks/index-6bcf98fc.js:1179:16)
2022-07-05T19:56:35.585364+00:00 app[web.1]:     at async respond (file:///app/apps/web-app/build/server/index.js:2110:22)
2022-07-05T19:56:35.585364+00:00 app[web.1]:     at async fetch (file:///app/apps/web-app/build/server/index.js:1466:22)

The thing is - we only see this error when deploying to Heroku! We can't reproduce locally - even when doing build + preview.

Where do I start debugging this - what should I look for? Any help would be very helpful.

We run node.js v16.