diff --git a/actionpack/lib/action_dispatch/routing/url_for.rb b/actionpack/lib/action_dispatch/routing/url_for.rb
index e836cf7c8e26f52d9153be7ae969f28e0dcaba4a..045c9ec77df76a8fb8f2dd76e584299a9f19df36 100644
--- a/actionpack/lib/action_dispatch/routing/url_for.rb
+++ b/actionpack/lib/action_dispatch/routing/url_for.rb
@@ -128,6 +128,7 @@ def url_for(options = nil)
         when String
           options
         when nil, Hash
+          @_routes ||= nil
           _routes.url_for((options || {}).reverse_merge!(url_options).symbolize_keys)
         else
           polymorphic_url(options)