diff --git a/lib/connect-mongodb.js b/lib/connect-mongodb.js
index dd477f1..a8c99f1 100644
--- a/lib/connect-mongodb.js
+++ b/lib/connect-mongodb.js
@@ -143,7 +143,7 @@ MONGOSTORE.prototype.set = function (sid, sess, cb) {
     update.expires = Date.parse(sess.cookie.expires);
   }
 
-  _collection.update({_id: sid}, update, {upsert: true}, function (err, data) {
+  _collection.update({_id: sid}, update, {upsert: true, safe: true}, function (err, data) {
     cb.apply(this, arguments);
   });
 };