fix: handle undefined statusText in writeHead arguments

index.js

@@ -120,7 +120,9 @@ function setWriteHeadHeaders (statusCode) {
   var length = arguments.length
   var headerIndex = length > 1 && typeof arguments[1] === 'string'
     ? 2
-    : 1
+    : length > 2
+      ? 2
+      : 1
 
   var headers = length >= headerIndex + 1
     ? arguments[headerIndex]

test/test.js

@@ -221,6 +221,34 @@ describe('onHeaders(res, listener)', function () {
     })
   })
 
+  describe('writeHead(status, undefined, obj)', function () {
+    it('should set headers when statusText is undefined', function (done) {
+      var server = createServer(echoListener, handler)
+
+      function handler (req, res) {
+        res.writeHead(200, undefined, { 'X-Outgoing': 'test' })
+      }
+
+      request(server)
+        .get('/')
+        .expect('X-Outgoing-Echo', 'test')
+        .expect(200, done)
+    })
+
+    it('should set headers when statusText is null', function (done) {
+      var server = createServer(echoListener, handler)
+
+      function handler (req, res) {
+        res.writeHead(200, null, { 'X-Outgoing': 'test' })
+      }
+
+      request(server)
+        .get('/')
+        .expect('X-Outgoing-Echo', 'test')
+        .expect(200, done)
+    })
+  })
+
   describe('writeHead(status, obj)', function () {
     it('should be available in listener', function (done) {
       var server = createServer(listener, handler)