diff --git a/typescript/core/utils/tree.ts b/typescript/core/utils/tree.ts
index 3ae195351..cfc773b27 100644
--- a/typescript/core/utils/tree.ts
+++ b/typescript/core/utils/tree.ts
@@ -26,7 +26,7 @@ export declare class Node {
 
 export declare class Tree {
   root: Node;
-  addNode(node: Node, newNode: Node, index: number):void;
+  addNode(node: Node, newNode: Node, index?: number):void;
   isRoot(node: Node): boolean;
   getRoot(): Node;
   clear():void;