diff --git a/lib/util/extend.js b/lib/util/extend.js
index 62da09e0..21476abf 100644
--- a/lib/util/extend.js
+++ b/lib/util/extend.js
@@ -1,7 +1,8 @@
 module.exports = function (base, opts) {
   var keys = Object.keys(opts)
   for (var i = 0; i < keys.length; ++i) {
-    base[keys[i]] = opts[keys[i]]
+    var k = keys[i]
+    base[k] = opts[k]
   }
   return base
 }