diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index 0aa7e3433ece5b1b13c9b84f9e41528ea91c1e1b..87e76113d6182b4a4f6756f02318944fb69d4700 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -124,11 +124,11 @@ def normalize_host(_host, options)
           return _host unless named_host?(_host)
 
           tld_length = options[:tld_length] || @@tld_length
-          subdomain  = options[:subdomain]
+          subdomain  = options.fetch :subdomain, true
           domain     = options[:domain]
 
           host = ""
-          if subdomain == true || !options.key?(:subdomain)
+          if subdomain == true
             return _host if domain.nil?
 
             host << extract_subdomain(_host, tld_length)