Be more friendly if the user goes ahead and adds devise_for :users before defining the model.

lib/devise/rails/routes.rb

@@ -93,6 +93,11 @@ module ActionDispatch::Routing
 
         begin
           raise_no_devise_method_error!(mapping.class_name) unless mapping.to.respond_to?(:devise)
+        rescue NameError => e
+          raise unless mapping.class_name == resource.to_s.classify
+          warn "[WARNING] You provided devise_for #{resource.inspect} but there is " <<
+            "no model #{mapping.class_name} defined in your application"
+          next
         rescue NoMethodError => e
           raise unless e.message.include?("undefined method `devise'")
           raise_no_devise_method_error!(mapping.class_name)