Observer: only walk object if it is plain

src/observer/index.js

@@ -25,7 +25,7 @@ function Observer (value) {
       : copyAugment
     augment(value, arrayMethods, arrayKeys)
     this.observeArray(value)
-  } else {
+  } else if (_.isPlainObject(value)) {
     this.walk(value)
   }
 }

src/util/lang.js

@@ -191,8 +191,9 @@ exports.isObject = function (obj) {
  */
 
 var toString = Object.prototype.toString
+var OBJECT_STRING = '[object Object]'
 exports.isPlainObject = function (obj) {
-  return toString.call(obj) === '[object Object]'
+  return toString.call(obj) === OBJECT_STRING
 }
 
 /**