diff --git a/types/NewWindow.d.ts b/types/NewWindow.d.ts
index 7a32ce8..0ab7542 100644
--- a/types/NewWindow.d.ts
+++ b/types/NewWindow.d.ts
@@ -13,7 +13,7 @@ declare module 'react-new-window' {
   export interface IWindowFeatures {
     height?: number
     width?: number
-    [i: string]: boolean | number | string
+    [i: string]: boolean | number | string | undefined
   }
 
   /**