Cannot add Vercel Toolbar to Next project

I’m following this guide, and am getting this error from Next. The URL referenced isn’t helping.

 ⚠ Invalid next.config.mjs options detected: 
 ⚠     Expected object, received string
 ⚠ See more info here: https://nextjs.org/docs/messages/invalid-next-config

My code looks like:

import withVercelToolbar from '@vercel/toolbar/plugins/next'

let nextConfig = {}

export default withVercelToolbar(nextConfig)

When combining with Sentry I get the error:

TypeError: Cannot set properties of undefined (setting 'clientTraceMetadata')