False alarm "You have added a custom /_error page without a custom /404"

[hems]

Bug report

I'm getting the error "Warning: You have added a custom /_error page without a custom /404 page. This prevents the 404 page from being auto statically optimized.
See here for info: https://err.sh/next.js/custom-error-no-custom-404" even tough i have both files in my pages/ folder.

Describe the bug

When running it locally with "yarn dev" i get that error even tough both files exists.

To Reproduce

# src/pages/404.js
import SEO from '../components/app/SEO'

function NotFound(props) {
  return (
    <>
      <SEO page='not-found' />
      <h1>Page Not Found</h1>
    </>
  );
}

export default NotFound;

# src/pages/_error.js
function Error({ statusCode }) {
  return (
    <>
      {statusCode
        ? `An error ${statusCode} occurred on server`
        : 'An error occurred on client'}
    </>
  )
}

Error.getInitialProps = ({ res, err }) => {
  const statusCode = res ? res.statusCode : err ? err.statusCode : 404
  return { statusCode }
}

export default Error

Expected behavior

Not get the error

System information

• OS: OSX 10.12
• Version of Next.js: 9.3.4
• Version of Node.js: 12.4

[balazsorban44]

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