diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 6a4b57e3f924b005166d3ed3e25fbb936841ca8f..526c97ff8e205c8c1a28baa1cb82ec73d464582d 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -152,7 +152,10 @@ def default_controller_and_action(to_shorthand=nil)
                 raise ArgumentError, "missing :action"
               end
 
-              { :controller => controller, :action => action }
+              { :controller => controller, :action => action }.tap do |hash|
+                hash.delete(:controller) if hash[:controller].blank?
+                hash.delete(:action)     if hash[:action].blank?
+              end
             end
           end