diff --git a/lib/_http_server.js b/lib/_http_server.js
index 99903024c26d14..6f1aad9e9eda0b 100644
--- a/lib/_http_server.js
+++ b/lib/_http_server.js
@@ -165,22 +165,16 @@ ServerResponse.prototype.writeHead = function(statusCode, reason, obj) {
   }
   this.statusCode = statusCode;
 
-  if (this._headers) {
-    // Slow-case: when progressive API and header fields are passed.
-    if (obj) {
-      var keys = Object.keys(obj);
-      for (var i = 0; i < keys.length; i++) {
-        var k = keys[i];
-        if (k) this.setHeader(k, obj[k]);
-      }
+  if (obj) {
+    var keys = Object.keys(obj);
+    for (var i = 0; i < keys.length; i++) {
+      var k = keys[i];
+      if (k) this.setHeader(k, obj[k]);
     }
-    // only progressive api is used
-    headers = this._renderHeaders();
-  } else {
-    // only writeHead() called
-    headers = obj;
   }
 
+  headers = this._renderHeaders();
+
   var statusLine = 'HTTP/1.1 ' + statusCode.toString() + ' ' +
                    this.statusMessage + CRLF;
 
diff --git a/test/parallel/test-http-mutable-headers.js b/test/parallel/test-http-mutable-headers.js
index 296b1622a03394..758484aee6dd79 100644
--- a/test/parallel/test-http-mutable-headers.js
+++ b/test/parallel/test-http-mutable-headers.js
@@ -54,6 +54,10 @@ var s = http.createServer(function(req, res) {
       res.setHeader('x-foo', 'keyboard cat');
       res.writeHead(200, { 'x-foo': 'bar', 'x-bar': 'baz' });
       break;
+
+    case 'writeHead2':
+      res.writeHead(200, { 'x-foo': 'bar', 'X-foo': 'bar', 'x-bar': 'baz' });
+      break;
   }
 
   res.statusCode = 201;
@@ -102,6 +106,13 @@ function nextTest() {
         break;
 
       case 'writeHead':
+        assert.equal(response.headers['x-foo'], 'bar');
+        assert.equal(response.headers['x-bar'], 'baz');
+        assert.equal(200, response.statusCode);
+        test = 'writeHead2';
+        break;
+
+      case 'writeHead2':
         assert.equal(response.headers['x-foo'], 'bar');
         assert.equal(response.headers['x-bar'], 'baz');
         assert.equal(200, response.statusCode);
@@ -109,7 +120,7 @@ function nextTest() {
         break;
 
       default:
-        throw Error('?');
+        throw new Error('?');
     }
 
     response.setEncoding('utf8');
@@ -127,6 +138,6 @@ function nextTest() {
 
 
 process.on('exit', function() {
-  assert.equal(4, testsComplete);
+  assert.equal(5, testsComplete);
 });