diff --git a/packages/next/src/lib/metadata/resolvers/resolve-basics.ts b/packages/next/src/lib/metadata/resolvers/resolve-basics.ts
index b10954e02756..1e69c4d61353 100644
--- a/packages/next/src/lib/metadata/resolvers/resolve-basics.ts
+++ b/packages/next/src/lib/metadata/resolvers/resolve-basics.ts
@@ -66,7 +66,10 @@ export const resolveViewport: FieldResolver<'viewport'> = (viewport) => {
 
 function resolveUrlValuesOfObject(
   obj:
-    | Record<string, string | URL | AlternateLinkDescriptor[] | null>
+    | Record<
+        string,
+        string | URL | AlternateLinkDescriptor[] | null | undefined
+      >
     | null
     | undefined,
   metadataBase: ResolvedMetadata['metadataBase'],
diff --git a/packages/next/src/lib/metadata/types/alternative-urls-types.ts b/packages/next/src/lib/metadata/types/alternative-urls-types.ts
index fd54da9c87a0..ac38a148076b 100644
--- a/packages/next/src/lib/metadata/types/alternative-urls-types.ts
+++ b/packages/next/src/lib/metadata/types/alternative-urls-types.ts
@@ -412,7 +412,15 @@ type LangCode =
   | 'zh-MO'
   | 'zh-SG'
   | 'zh-TW'
+  | 'zh-Hans'
+  | 'zh-Hant'
   | 'zu-ZA'
+  // We can't have all valid combinations of language-region-script listed here
+  // as the list is too long and breaks the TypeScript compiler. So instead we
+  // only add the most common ones with a general string pattern for the rest.
+  // This way autocompletion still works and it is still possible to add custom
+  // lang codes.
+  | `${Lowercase<string>}-${string}`
 
 type UnmatchedLang = 'x-default'