---
title: "`generateBuildId` did not return a string"
url: "https://nextjs.org/docs/messages/generatebuildid-not-a-string"
docs_index: /docs/llms.txt
---



## Why This Error Occurred

The most common cause for this issue is a custom `next.config.js` with the `generateBuildId` method defined, but it does not return a string.

## Possible Ways to Fix It

Always return a string from generateBuildId.