check the type of transaction

lib/query-interface.js

 var Utils                = require(__dirname + '/utils')
   , DataTypes            = require(__dirname + '/data-types')
   , SQLiteQueryInterface = require(__dirname + '/dialects/sqlite/query-interface')
+  , Transaction          = require(__dirname + '/transaction')
 
 module.exports = (function() {
   var QueryInterface = function(sequelize) {
@@ -799,7 +800,7 @@ module.exports = (function() {
   }
 
   QueryInterface.prototype.startTransaction = function(transaction, options) {
-    if (!transaction) {
+    if (!transaction || !(transaction instanceof Transaction)) {
       throw new Error('Unable to start a transaction without transaction object!')
     }
 
@@ -812,7 +813,7 @@ module.exports = (function() {
   }
 
   QueryInterface.prototype.commitTransaction = function(transaction, options) {
-    if (!transaction) {
+    if (!transaction || !(transaction instanceof Transaction)) {
       throw new Error('Unable to commit a transaction without transaction object!')
     }
 
@@ -825,7 +826,7 @@ module.exports = (function() {
   }
 
   QueryInterface.prototype.rollbackTransaction = function(transaction, options) {
-    if (!transaction) {
+    if (!transaction || !(transaction instanceof Transaction)) {
       throw new Error('Unable to rollback a transaction without transaction object!')
     }