diff --git a/lib/index.js b/lib/index.js
index 76f9957..982f2ae 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -92,7 +92,7 @@ exports.encode = function(opts) {
 }
 
 var Encoder = exports.Encoder = function(opts) {
-  this.opts = util._extend({ lengthSize: 4 }, opts)
+  this.opts = Object.assign({ lengthSize: 4 }, opts)
 
   this.setLength = this.opts.setLength || createSetLengthMethod(this.opts.lengthSize)