diff --git a/react-portal/index.d.ts b/react-portal/index.d.ts
index 0418d576bc..2d4bd8721e 100644
--- a/react-portal/index.d.ts
+++ b/react-portal/index.d.ts
@@ -2,6 +2,7 @@
 // Project: https://github.com/tajo/react-portal#readme
 // Definitions by: Shun Takahashi <https://github.com/shuntksh>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.1
 
 import * as React from "react";