diff --git a/package-lock.json b/package-lock.json
index dc62329..200d454 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@iden3/js-merkletree",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@iden3/js-merkletree",
-      "version": "1.3.0",
+      "version": "1.3.1",
       "license": "AGPL-3.0",
       "devDependencies": {
         "@iden3/eslint-config": "https://github.com/iden3/eslint-config",
diff --git a/package.json b/package.json
index 4da6f27..6eeb347 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@iden3/js-merkletree",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "description": "javascript sparse merkle tree library",
   "types": "dist/types/index.d.ts",
   "main": "dist/node/cjs/index.js",
diff --git a/src/lib/merkletree/proof.ts b/src/lib/merkletree/proof.ts
index 23c8f1d..2f2c8ba 100644
--- a/src/lib/merkletree/proof.ts
+++ b/src/lib/merkletree/proof.ts
@@ -10,11 +10,11 @@ import { Bytes } from '../../types';
 export interface ProofJSON {
   existence: boolean;
   siblings: string[];
-  node_aux: NodeAuxJSON | undefined; // this is a right representation of auxiliary node field according to the specification, nodeAux will be deprecated.
+  node_aux?: NodeAuxJSON; // this is a right representation of auxiliary node field according to the specification, nodeAux will be deprecated.
   /**
    * @deprecated old version is deprecated, do not use it.
    */
-  nodeAux: NodeAuxJSON | undefined; // old version of representation of auxiliary node.
+  nodeAux?: NodeAuxJSON; // old version of representation of auxiliary node.
 }
 
 export interface NodeAuxJSON {