diff --git a/lib/util.js b/lib/util.js
index c6c543d5b3350b..5a16076b7c8415 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -1,5 +1,7 @@
 'use strict';
 
+const Url = require('url').Url;
+
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
   if (typeof f !== 'string') {
@@ -632,7 +634,8 @@ exports.inherits = function(ctor, superCtor) {
 exports._extend = function(origin, add) {
   // Don't do anything if add isn't an object
   if (add === null || typeof add !== 'object') return origin;
-  if (typeof add.toJSON === 'function') add = add.toJSON();
+  // For compatibility with Url objects
+  if (add instanceof Url) add = add.toJSON();
 
   var keys = Object.keys(add);
   var i = keys.length;