diff --git a/webpack/index.d.ts b/webpack/index.d.ts
index 123f1f09b3..b31e168dc5 100644
--- a/webpack/index.d.ts
+++ b/webpack/index.d.ts
@@ -209,7 +209,7 @@ declare namespace webpack {
         Buffer?: boolean;
         __filename?: boolean | string;
         __dirname?: boolean | string;
-        [nodeBuiltin: string]: boolean | string;
+        [nodeBuiltin: string]: boolean | string | undefined;
     }
 
     type LoaderCondition = string | RegExp | ((absPath: string) => boolean);