add fn assertion to app.use(). Closes #337

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;