diff --git a/lib/buffer.js b/lib/buffer.js
index f6f28294e3c3c7..3d5e96e52f3042 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -276,6 +276,7 @@ function byteLength(string, encoding) {
 
       case 'utf8':
       case 'utf-8':
+      case undefined:
         return binding.byteLengthUtf8(string);
 
       case 'ucs2':