diff --git a/express-serve-static-core/express-serve-static-core.d.ts b/express-serve-static-core/express-serve-static-core.d.ts
index 624ed5ae5d..0371c2bb6a 100644
--- a/express-serve-static-core/express-serve-static-core.d.ts
+++ b/express-serve-static-core/express-serve-static-core.d.ts
@@ -22,7 +22,7 @@ declare module "express-serve-static-core" {
     }
 
     interface RequestHandler {
-        (req: Request, res: Response, next?: NextFunction): any;
+        (req: Request, res: Response, next: NextFunction): any;
     }
 
     interface ErrorRequestHandler {
@@ -822,6 +822,12 @@ declare module "express-serve-static-core" {
     }
 
     interface Application extends IRouter, Express.Application {
+        /**
+         * Express instance itself is a request handler, which could be invoked without
+         * third argument.
+         */
+        (req: Request, res: Response): any;
+        
         /**
             * Initialize the server.
             *