diff --git a/lib/types/objectid.js b/lib/types/objectid.js
index cbe9bd8b06a..692799e0994 100644
--- a/lib/types/objectid.js
+++ b/lib/types/objectid.js
@@ -24,4 +24,8 @@ Object.defineProperty(ObjectId.prototype, '_id', {
   }
 });
 
+ObjectId.prototype.valueOf = function() {
+  return this.toString();
+};
+
 module.exports = ObjectId;