diff --git a/lib/Tsifier.js b/lib/Tsifier.js
index 8e84002..ebc9f4b 100644
--- a/lib/Tsifier.js
+++ b/lib/Tsifier.js
@@ -2,7 +2,7 @@
 
 var convert  = require('convert-source-map');
 var events   = require('events');
-var extend   = require('util')._extend;
+var extend   = Object.assign;
 var fs       = require('fs');
 var realpath = require('fs.realpath');
 var log      = require('util').debuglog(require('../package').name);