diff --git a/.changeset/cool-planets-flow.md b/.changeset/cool-planets-flow.md
new file mode 100644
index 0000000000..dfbc21cbef
--- /dev/null
+++ b/.changeset/cool-planets-flow.md
@@ -0,0 +1,7 @@
+---
+'nextra': patch
+'nextra-theme-blog': patch
+'nextra-theme-docs': patch
+---
+
+fix: avoid mutating nextConfig
diff --git a/packages/nextra/src/index.js b/packages/nextra/src/index.js
index 524e26409b..6c65911a3c 100644
--- a/packages/nextra/src/index.js
+++ b/packages/nextra/src/index.js
@@ -4,37 +4,37 @@ import { DEFAULT_LOCALE, MARKDOWN_EXTENSION_REGEX } from './constants'
 const DEFAULT_EXTENSIONS = ['js', 'jsx', 'ts', 'tsx']
 const MARKDOWN_EXTENSIONS = ['md', 'mdx']
 
-const nextra = (...args) =>
+const nextra = (...config) =>
   function withNextra(nextConfig = {}) {
     const nextraConfig =
-      typeof args[0] === 'string'
+      typeof config[0] === "string"
         ? {
-            theme: args[0],
-            themeConfig: args[1]
+            theme: config[0],
+            themeConfig: config[1]
           }
-        : args[0]
+        : config[0]
 
     const nextraPlugin = new NextraPlugin(nextraConfig)
-    const { i18n, pageExtensions = DEFAULT_EXTENSIONS } = nextConfig
+    const { pageExtensions = DEFAULT_EXTENSIONS } = nextConfig
 
-    if (i18n?.locales) {
+    if (nextConfig.i18n?.locales) {
       console.log(
         '[nextra] You have Next.js i18n enabled, read here https://nextjs.org/docs/advanced-features/i18n-routing for the docs.'
       )
-    } else if (!i18n?.defaultLocale) {
-      // If `i18n.locales` and `i18n.defaultLocale` were not specified,
-      // client will receive error - Text content does not match server-rendered HTML.
-      // Due to `const { locale } = useRouter()` where `locale` will be `undefined`
-      // To fix it we need to explicitly specify `i18n.locales` and `i18n.defaultLocale`
-      nextConfig.i18n = {
-        ...i18n,
-        locales: [DEFAULT_LOCALE],
-        defaultLocale: DEFAULT_LOCALE
-      }
+    }
+
+    // If `i18n.locales` and `i18n.defaultLocale` were not specified,
+    // client will receive error - Text content does not match server-rendered HTML.
+    // Due to `const { locale } = useRouter()` where `locale` will be `undefined`
+    // To fix it we need to explicitly specify `i18n.locales` and `i18n.defaultLocale`
+    const i18n = {
+      locales: nextConfig.i18n?.locales || [DEFAULT_LOCALE],
+      defaultLocale: nextConfig.i18n?.defaultLocale || DEFAULT_LOCALE
     }
 
     return {
       ...nextConfig,
+      i18n,
       pageExtensions: [...pageExtensions, ...MARKDOWN_EXTENSIONS],
       webpack(config, options) {
         config.plugins ||= []
@@ -48,8 +48,8 @@ const nextra = (...args) =>
               loader: 'nextra/loader',
               options: {
                 ...nextraConfig,
-                locales: nextConfig.i18n?.locales,
-                defaultLocale: nextConfig.i18n?.defaultLocale,
+                locales: i18n.locales,
+                defaultLocale: i18n.defaultLocale,
                 pageMapCache
               }
             }
@@ -64,4 +64,5 @@ const nextra = (...args) =>
       }
     }
   }
+
 module.exports = nextra