diff --git a/koa/index.d.ts b/koa/index.d.ts
index 7fa9dd1cc9..3d787b15b3 100644
--- a/koa/index.d.ts
+++ b/koa/index.d.ts
@@ -43,6 +43,8 @@ declare namespace Koa {
 
         toJSON(): any;
         inspect(): any;
+
+        [key: string]: any;
     }
 
     export interface Request {