diff --git a/lib/base64-arraybuffer.js b/lib/base64-arraybuffer.js
index e6b6306..3ad6161 100644
--- a/lib/base64-arraybuffer.js
+++ b/lib/base64-arraybuffer.js
@@ -16,8 +16,8 @@
     lookup[chars.charCodeAt(i)] = i;
   }
 
-  exports.encode = function(arraybuffer) {
-    var bytes = new Uint8Array(arraybuffer),
+  exports.encode = function(arraybuffer, offset, length) {
+    var bytes = new Uint8Array(arraybuffer, offset || 0, length !== undefined ? length : arraybuffer.byteLength),
     i, len = bytes.length, base64 = "";
 
     for (i = 0; i < len; i+=3) {