diff --git a/lib/middleware.js b/lib/middleware.js
index 41cf828..c9fff9c 100644
--- a/lib/middleware.js
+++ b/lib/middleware.js
@@ -26,12 +26,16 @@ module.exports.create = function (le) {
     return function (req, res, next) {
       if (0 !== req.url.indexOf(prefix)) {
         log(le.debug, "no match, skipping middleware");
-        if (_app) {
+        if ('function' === typeof _app) {
           _app(req, res, next);
         }
-        else {
+        else if ('function' === typeof next) {
           next();
         }
+        else {
+          res.statusCode = 500;
+          res.end("[500] Developer Error: app.use('/', le.middleware()) or le.middleware(app)");
+        }
         return;
       }