diff --git a/lib/application.js b/lib/application.js
index c59ab9d..9be3123 100644
--- a/lib/application.js
+++ b/lib/application.js
@@ -96,7 +96,7 @@ app.toJSON = function(){
  */
 
 app.use = function(fn){
-  assert('GeneratorFunction' == fn.constructor.name, 'app.use() requires a generator function');
+  assert(fn && 'GeneratorFunction' == fn.constructor.name, 'app.use() requires a generator function');
   debug('use %s', fn._name || fn.name || '-');
   this.middleware.push(fn);
   return this;