diff --git a/lib/buffer.js b/lib/buffer.js
index 210fdf4f05..dff740e1b2 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -272,6 +272,7 @@ function byteLength(string, encoding) {
 
       case 'utf8':
       case 'utf-8':
+      case undefined:
         return binding.byteLengthUtf8(string);
 
       case 'ucs2':