Fix: TypeError: Cannot read properties of null (reading 'auth') on Next.js \_not-found

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You're deploying to Vercel, feeling confident about your latest changes. Then the build fails with this cryptic error:

Error occurred prerendering page "/_not-found"
TypeError: Cannot read properties of null (reading 'auth')

If you're hitting this Next.js prerender error _not-found auth issue, you're not alone. This error has tripped up many developers, especially those using authentication wrappers in their root layouts. Let's break down what's happening and how to fix it properly.

The Problem — Build Fails on Vercel with 'TypeError: Cannot read properties of null (reading auth)' for _not-found

The error typically surfaces during next build when Next.js attempts to statically prerender the /_not-found page. The full error often looks something like this:

Generating static pages (0/8) [= ]
Error occurred prerendering page "/_not-found".
Read more: https://nextjs.org/docs/messages/prerender-error

TypeError: Cannot read properties of null (reading 'auth')
    at RootLayout (/app/layout.tsx:25:32)
    ...

> Export encountered errors on following paths:
    /_not-found/page: /_not-found

This issue gained significant attention in GitHub issue #65447, where developers reported similar failures after upgrading to Next.js 14.2.x, particularly when using the pageExtensions configuration option. However, the root cause isn't limited to pageExtensions—it's fundamentally about accessing authentication context during static prerendering.

The frustrating part? Your app works perfectly during . It only blows up when you try to build for production.

Why This Happens — Attempting to Access Authentication Context in a Static Page That Lacks Session Data During Build

To understand this error, you need to grasp how Next.js handles the _not-found page differently from your other routes.

Static Prerendering in Next.js

During next build, Next.js prerenders certain pages as static HTML to optimize performance. The _not-found page is one of these. It gets generated at build time—without any request context, session data, or authentication information.

Here's the critical insight: there is no user session during build time. No cookies, no headers, no authenticated user. It's just Next.js generating static HTML on your CI server.

The Auth Wrapper Problem

Many applications wrap their root layout with an authentication provider or access session data directly. A common pattern looks like this:

When Next.js prerenders /_not-found, it renders your root layout.tsx as part of the page tree. If that layout calls auth() and immediately tries to access properties on the result without checking for null, you get the dreaded TypeError.

The pageExtensions Complication

The GitHub issue #65447 specifically mentions pageExtensions because this configuration can affect how Next.js discovers and processes special files like not-found.tsx. When you use custom page extensions (e.g., ['page.tsx', 'page.ts']), Next.js might not properly resolve your custom not-found.page.tsx file, falling back to an internal _not-found route that still renders through your root layout.

The combination of:

1. Custom pageExtensions configuration
2. Auth wrappers in the root layout without null safety
3. The _not-found page being statically prerendered

Creates the perfect storm for this Next.js prerender error _not-found auth failure.

The Solution — Add Null-Checks or Use Safe Optional Chaining When Accessing Auth Properties in Global Layouts

The fix is straightforward once you understand the problem: never assume authentication data exists in components that render during static builds.

Approach 1: Optional Chaining

The simplest fix is using optional chaining (?.) to safely access potentially null properties:

Approach 2: Early Return Guard

For more complex auth-dependent UI, use an early return pattern:

Approach 3: Isolate Auth Logic in Client Components

If your authentication UI is complex, consider moving it to a client component that handles its own loading state:

This approach moves the auth logic entirely to the client, avoiding the prerender issue altogether. The trade-off is a slight flash of loading state on initial page load.

Fixing pageExtensions-Related Issues

If you're using pageExtensions and encountering this issue, ensure your special files follow the naming convention:

With this config, your not-found file should be named not-found.page.tsx:

app/
├── layout.page.tsx
├── page.page.tsx
├── not-found.page.tsx    // Must match pageExtensions
└── error.page.tsx

Important: Even with correct file naming, you still need the null-safety patterns above. The file naming just ensures Next.js finds your custom 404 page.

Prevention — Testing Production Builds Locally with 'next build' to Catch Prerendering Errors Before Deployment

The best defense against prerender errors is catching them before they hit your CI pipeline. Here's a prevention checklist:

Run Production Builds Locally

Make it a habit to run next build locally before pushing:

This will surface any prerender errors immediately, saving you the round-trip to Vercel.

Create a Pre-Push Hook

Automate the check with a git hook using Husky:

Add a CI Step

For team projects, add an explicit build step in your CI:

Audit Your Root Layout

Review your root layout.tsx for any assumptions about request-time data:

• Session/auth data: Always use optional chaining or guards
• Headers/cookies: Not available during static generation
• Request URL: Use with caution, may be undefined during build

Create a Defensive Auth Wrapper

Consider creating a reusable wrapper that handles null cases:

Using this in your layout:

This pattern centralizes your null-safety logic and makes it reusable across your application.

Key Takeaways

• The _not-found page is statically prerendered during next build, meaning there's no session, cookies, or request context available.

• Auth wrappers in root layouts must handle null cases using optional chaining (?.), guard clauses, or by isolating auth logic in client components.

• The pageExtensions config can complicate things by affecting how Next.js discovers special files like not-found.tsx. Ensure your files match the configured extensions.

• Always run next build locally before deploying to catch Next.js prerender error _not-found auth issues early, rather than debugging them in your CI logs.

• Create defensive auth utilities that gracefully handle undefined sessions, making your codebase resilient to static generation edge cases.

Next Steps

1. Audit your root layout right now. Search for any direct property access on session or auth objects without null checks.

2. Run next build locally to verify your fixes work before pushing to your CI.

3. Set up pre-push hooks or CI checks to catch prerender errors automatically.

4. Review the GitHub issue #65447 for community workarounds if you're dealing with the pageExtensions variant of this problem.

5. Consider the client component approach if your auth UI is complex—it trades a loading flash for complete immunity from prerender issues.

The Next.js prerender error _not-found auth issue is ultimately about understanding the boundary between static and dynamic rendering. Once you internalize that build-time rendering has no request context, defensive coding becomes second nature. Your future self—and your CI pipeline—will thank you.