diff --git a/index.js b/index.js
index 167419d..fc723eb 100644
--- a/index.js
+++ b/index.js
@@ -467,12 +467,12 @@ class API {
     let middleware = [];
 
     // Add func args as middleware
-    for (let arg in args) {
-      if (typeof args[arg] === 'function') {
-        if (args[arg].length === 3) {
-          middleware.push(args[arg]);
-        } else if (args[arg].length === 4) {
-          this._errors.push(args[arg]);
+    for (let arg of args) {
+      if (typeof arg === 'function') {
+        if (arg.length === 3) {
+          middleware.push(arg);
+        } else if (arg.length === 4) {
+          this._errors.push(arg);
         } else {
           throw new ConfigurationError(
             'Middleware must have 3 or 4 parameters'