Looks like the results of runtime_config_default()() is undefined.
In another project the build works fine, but I can't find runtime_config_default inside .next.

It seems to be related to lerna & different not being installed correctly, installing it in the root of the project with yarn add next -W fixed it for me.

I installed a package & this error is back, also in dev mode! Also tried 9.5.1.

Oke, so I found the situation when this is happening. I use next-transpile-modules to import modules from a different component package. But when I try to call getConfig inside a component in a different package it fails, a workaround is passing the config to the Component.

I encountered this same error when using yarn workspaces with one workspace depending on next 9.4 and another 9.5. updating next to the latest version in all workspaces resolved it for me.

I've got the same error too, when I've introduced getStaticProps.
it occurred when next dev subcommand executed, but the next build && next start did not the case.

Version of Next.js: 9.5.1

I am geting this error in Next 9.5.5 under jest tests, just checking if my component isDefined

I started to get this error when I updated from v10.0.6 to v10.0.7

I'm getting that error too I'm on v10.0.7 but had it since v9.5

Oke, so I found the situation when this is happening. I use next-transpile-modules to import modules from a different component package. But when I try to call getConfig inside a component in a different package it fails, a workaround is passing the config to the Component.

I also have next-transpile-modules in my next.config.js and running into this issue when running my tests? Any recommendations?

I had this issue with npm workspaces. turns out i was calling next from the workspace node_modules and not the correct node_modules from the package folder. Basically I was calling the wrong version.

@gushiguera I have the same situation as you. What I did is mock it with jest.

I solved this issue with this lines in jest.setup file:

import { setConfig } from 'next/config'
import config from './next.config'

setConfig(config)

For those who encounter this running jest, here is a fix tested
In my case I was using

import getConfig from 'next/config';
const { publicRuntimeConfig } = getConfig();

Changing the above to import { publicRuntimeConfig } from '<path/to>/next.config.js'; helped