diff --git a/dns.types.js b/dns.types.js
index b3aaa31..e10788d 100644
--- a/dns.types.js
+++ b/dns.types.js
@@ -49,8 +49,8 @@ var types = exports.DNS_TYPES = {
 };
 
 // and in reverse
-for (var key in types) {
+Object.keys(types).forEach(function (key) {
   types[types[key]] = key;
-}
+});
 
 }('undefined' !== typeof window ? window : exports));