diff --git a/types/koa-mount/index.d.ts b/types/koa-mount/index.d.ts
index 138d4f6f36..85f1220892 100644
--- a/types/koa-mount/index.d.ts
+++ b/types/koa-mount/index.d.ts
@@ -1,15 +1,21 @@
-// Type definitions for koa-mount 3.0
+// Type definitions for koa-mount 4.0
 // Project: https://github.com/koajs/mount
 // Definitions by: AmirSaber Sharifi <https://github.com/amirsaber>
 //                 Tomek Łaziuk <https://github.com/tlaziuk>
+//                 Vladislav Polyakov <https://github.com/polrk>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
 import Koa = require("koa");
 
-declare function mount(app: Koa.Middleware | Koa): Koa.Middleware;
+declare function mount<StateT = any, CustomT = {}>(
+  app: Koa.Middleware<StateT, CustomT> | Koa<StateT, CustomT>
+): Koa.Middleware<StateT, CustomT>;
 
-declare function mount(prefix: string, app: Koa.Middleware | Koa): Koa.Middleware;
+declare function mount<StateT = any, CustomT = {}>(
+  prefix: string,
+  app: Koa.Middleware<StateT, CustomT> | Koa<StateT, CustomT>
+): Koa.Middleware<StateT, CustomT>;
 
 declare namespace mount { }