diff --git a/index.d.ts b/index.d.ts
index c38027e..aa9f577 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -26,6 +26,7 @@ declare interface IConfig {
     cert: string;
   };
   readonly generateClientId?: () => string;
+  readonly createWebSocketServer?: (options: WebSocketLib.ServerOptions) => WebSocketLib.Server;
 }
 
 declare interface IClient {