fixes 404 no longer returned when error with code ENOENT is thrown.

packages/next/next-server/server/next-server.ts

@@ -1237,9 +1237,14 @@ export default class Server {
         }
       }
     } catch (err) {
-      this.logError(err)
-      res.statusCode = 500
-      return await this.renderErrorToHTML(err, req, res, pathname, query)
+      if (err && err.code === 'ENOENT') {
+        res.statusCode = 404
+        return await this.renderErrorToHTML(null, req, res, pathname, query)
+      } else {
+        this.logError(err)
+        res.statusCode = 500
+        return await this.renderErrorToHTML(err, req, res, pathname, query)
+      }
     }
 
     res.statusCode = 404

packages/next/next-server/server/render.tsx

@@ -637,7 +637,7 @@ export async function renderToHTML(
       ;(renderOpts as any).pageData = props
     }
   } catch (err) {
-    if (isDataReq || !dev || !err) throw err
+    if (isDataReq || !dev || !err || err.code === 'ENOENT') throw err
     ctx.err = err
     renderOpts.err = err
     console.error(err)

test/integration/custom-error/pages/throw-404.js

@@ -0,0 +1,9 @@
+const Page = () => 'hi'
+
+Page.getInitialProps = () => {
+  const error = new Error('to 404 we go')
+  error.code = 'ENOENT'
+  throw error
+}
+
+export default Page

test/integration/custom-error/test/index.test.js

@@ -17,10 +17,16 @@ const nextConfig = join(appDir, 'next.config.js')
 let appPort
 let app
 
-const runTests = () => {
+const runTests = isDev => {
   it('renders custom _error successfully', async () => {
     const html = await renderViaHTTP(appPort, '/')
-    expect(html).toMatch(/Custom error/)
+    expect(html).toMatch(isDev ? /oof/ : /Custom error/)
+  })
+
+  it('renders 404 page when ENOENT error is thrown', async () => {
+    const html = await renderViaHTTP(appPort, '/throw-404')
+    expect(html).toContain('Custom error')
+    expect(html).not.toContain('to 404 we go')
   })
 }
 
@@ -56,6 +62,8 @@ describe('Custom _error', () => {
       expect(html).toContain('An error 404 occurred on server')
       expect(stderr).toMatch(customErrNo404Match)
     })
+
+    runTests(true)
   })
 
   describe('production mode', () => {