diff --git a/src/jsmind.util.js b/src/jsmind.util.js
index bf1b2c22..3e40b66d 100644
--- a/src/jsmind.util.js
+++ b/src/jsmind.util.js
@@ -74,6 +74,9 @@ export const util = {
         },
         merge: function (b, a) {
             for (var o in a) {
+                if (o === '__proto__' || o === 'constructor' || o === 'prototype') {
+                    continue;
+                }
                 if (o in b) {
                     if (
                         typeof b[o] === 'object' &&