diff --git a/HISTORY.md b/HISTORY.md
index 6c8a4e0..92bbe7e 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -3,6 +3,12 @@
 https://github.com/josdejong/jsoneditor
 
 
+## not yet released, version 5.5.9
+
+- Fixed #329: Editor showing duplicate key warnings for keys defined on the
+  Object prototype, like `toString` and `watch`.
+
+
 ## 2016-09-27, version 5.5.8
 
 - Fixed #314: JSON schema validation throwing an error "Unexpected token ' in
diff --git a/src/js/Node.js b/src/js/Node.js
index f7bcd8d..ad6592b 100644
--- a/src/js/Node.js
+++ b/src/js/Node.js
@@ -1416,7 +1416,7 @@ Node.prototype.validate = function () {
     var duplicateKeys = [];
     for (var i = 0; i < this.childs.length; i++) {
       var child = this.childs[i];
-      if (keys[child.field]) {
+      if (keys.hasOwnProperty(child.field)) {
         duplicateKeys.push(child.field);
       }
       keys[child.field] = true;