diff --git a/packages/next/client/index.js b/packages/next/client/index.js
index 4292764b202ec..3a02f15adb24c 100644
--- a/packages/next/client/index.js
+++ b/packages/next/client/index.js
@@ -9,7 +9,7 @@ import { RouterContext } from '../next-server/lib/router-context'
 import { isDynamicRoute } from '../next-server/lib/router/utils/is-dynamic'
 import * as envConfig from '../next-server/lib/runtime-config'
 import { getURL, loadGetInitialProps, ST } from '../next-server/lib/utils'
-import { delBasePath, basePath } from '../next-server/lib/router/router'
+import { delBasePath, hasBasePath } from '../next-server/lib/router/router'
 import initHeadManager from './head-manager'
 import PageLoader from './page-loader'
 import measureWebVitals from './performance-relayer'
@@ -52,7 +52,7 @@ envConfig.setConfig({
 let asPath = getURL()
 
 // make sure not to attempt stripping basePath for 404s
-if (asPath.startsWith(basePath)) {
+if (hasBasePath(asPath)) {
   asPath = delBasePath(asPath)
 }
 
diff --git a/packages/next/next-server/lib/router/router.ts b/packages/next/next-server/lib/router/router.ts
index 538db7544ff6b..f3283f29194bb 100644
--- a/packages/next/next-server/lib/router/router.ts
+++ b/packages/next/next-server/lib/router/router.ts
@@ -22,7 +22,7 @@ import {
   normalizePathTrailingSlash,
 } from '../../../client/normalize-trailing-slash'
 
-export const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
+const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
 
 function buildCancellationError() {
   return Object.assign(new Error('Route Cancelled'), {
@@ -30,6 +30,10 @@ function buildCancellationError() {
   })
 }
 
+export function hasBasePath(path: string): boolean {
+  return path === basePath || path.startsWith(basePath + '/')
+}
+
 export function addBasePath(path: string): string {
   return basePath
     ? path === '/'
@@ -268,7 +272,7 @@ export default class Router implements BaseRouter {
       const browserUrl = getURL()
       // make sure "as" doesn't start with double slashes or else it can
       // throw an error as it's considered invalid
-      if (as.substr(0, 2) !== '//' && browserUrl.startsWith(basePath)) {
+      if (as.substr(0, 2) !== '//' && hasBasePath(browserUrl)) {
         // in order for `e.state` to work on the `onpopstate` event
         // we have to register the initial route upon initialization
         // if it doesn't start with the basePath, it's to be treated as an external url
@@ -335,14 +339,17 @@ export default class Router implements BaseRouter {
       // Actually, for (1) we don't need to nothing. But it's hard to detect that event.
       // So, doing the following for (1) does no harm.
       const browserUrl = getURL()
-      if (browserUrl.startsWith(basePath)) {
-        // if it doesn't start with the basePath, it's to be treated as an external url
+      console.log(document.location.href, e)
+      if (hasBasePath(browserUrl)) {
         const { pathname, query } = this
         this.changeState(
           'replaceState',
           formatWithValidation({ pathname, query }),
           delBasePath(browserUrl)
         )
+      } else {
+        // if it doesn't start with the basePath, it's to be treated as an external url
+        window.location.reload()
       }
       return
     }
diff --git a/test/integration/basepath/test/index.test.js b/test/integration/basepath/test/index.test.js
index 593d9b23e95e7..be3df18cb2705 100644
--- a/test/integration/basepath/test/index.test.js
+++ b/test/integration/basepath/test/index.test.js
@@ -216,6 +216,27 @@ const runTests = (context, dev = false) => {
     expect(pathname).toBe('/missing')
   })
 
+  it('should handle 404 urls that start with basePath', async () => {
+    const browser = await webdriver(context.appPort, '/docshello')
+    expect(await browser.eval(() => window.next.router.asPath)).toBe(
+      '/docshello'
+    )
+    expect(await browser.eval(() => window.location.pathname)).toBe(
+      '/docshello'
+    )
+  })
+
+  it('should navigating back to a non-basepath 404 that starts with basepath', async () => {
+    const browser = await webdriver(context.appPort, '/docshello')
+    await browser.eval(() => window.next.router.push('/hello'))
+    await browser.waitForElementByCss('#pathname')
+    await browser.back()
+    check(() => browser.eval(() => window.location.pathname), '/docshello')
+    expect(await browser.eval(() => window.next.router.asPath)).toBe(
+      '/docshello'
+    )
+  })
+
   it('should update dynamic params after mount correctly', async () => {
     const browser = await webdriver(context.appPort, '/docs/hello-dynamic')
     const text = await browser.elementByCss('#slug').text()
diff --git a/test/integration/build-output/test/index.test.js b/test/integration/build-output/test/index.test.js
index 2b4f1738042b6..01adfb3cb6703 100644
--- a/test/integration/build-output/test/index.test.js
+++ b/test/integration/build-output/test/index.test.js
@@ -104,7 +104,7 @@ describe('Build Output', () => {
       expect(parseFloat(err404FirstLoad) - 63).toBeLessThanOrEqual(0)
       expect(err404FirstLoad.endsWith('kB')).toBe(true)
 
-      expect(parseFloat(sharedByAll) - 59.3).toBeLessThanOrEqual(0)
+      expect(parseFloat(sharedByAll) - 59.4).toBeLessThanOrEqual(0)
       expect(sharedByAll.endsWith('kB')).toBe(true)
 
       if (_appSize.endsWith('kB')) {