diff --git a/lib/middleman/base.rb b/lib/middleman/base.rb
index 62975da8..3134e5c0 100644
--- a/lib/middleman/base.rb
+++ b/lib/middleman/base.rb
@@ -34,7 +34,7 @@ class Middleman::Base
     end
     
     def prototype
-      @prototype ||= app.to_app
+      @prototype ||= to_rack_app
     end
 
     def call(env)