---
title: 'The key "<your key>" under "env" in `next.config.js` is not allowed.'
---

## Why This Error Occurred

Next.js configures internal variables for replacement itself. `NEXT_RUNTIME` along with variables starting with `__` or `NODE_` are currently internal, for this reason they are not allowed as values for `env` in `next.config.js`

## Possible Ways to Fix It

Rename the specified key so that it does not start with `__` or `NODE_`, or pick another name for `NEXT_RUNTIME`.