---
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`.