diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 84a08770f5af414aa4b5ff57f560869d62947fd5..68f9225a45becb20b51ac256a9cda90569494bd5 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -240,15 +240,18 @@ def default_controller_and_action
             if to.respond_to?(:call)
               { }
             else
-              if to.is_a?(String)
-                controller, action = to.split('#')
-              elsif to.is_a?(Symbol)
+              controller = default_controller
+              action     = default_action
+
+              case to
+              when Symbol
                 action = to.to_s
+              when /#/
+                controller, action = to.split('#')
+              when String
+                controller = to
               end
 
-              controller ||= default_controller
-              action     ||= default_action
-
               if @scope[:module] && !controller.is_a?(Regexp)
                 if controller =~ %r{\A/}
                   controller = controller[1..-1]