diff --git a/lib/url.js b/lib/url.js
index bb11144f7a33bc..3c3e91d2a99189 100644
--- a/lib/url.js
+++ b/lib/url.js
@@ -548,7 +548,7 @@ Url.prototype.format = function() {
   var pathname = this.pathname || '';
   var hash = this.hash || '';
   var host = false;
-  var query = '';
+  var query;
 
   if (this.host) {
     host = auth + this.host;
@@ -563,6 +563,8 @@ Url.prototype.format = function() {
 
   if (this.query !== null && typeof this.query === 'object')
     query = querystring.stringify(this.query);
+  else
+    query = this.query;
 
   var search = this.search || (query && ('?' + query)) || '';
 
diff --git a/test/parallel/test-url.js b/test/parallel/test-url.js
index 760303e98bb6bb..1e3f923e2741c6 100644
--- a/test/parallel/test-url.js
+++ b/test/parallel/test-url.js
@@ -1586,3 +1586,6 @@ for (let i = 0; i < throws.length; i++) {
 }
 assert(url.format('') === '');
 assert(url.format({}) === '');
+
+// https://github.com/nodejs/node/issues/6004
+assert.equal(url.format({pathname:'/foo', query: 'bar=baz'}), '/foo?bar=baz');