diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index 278f53438028074cc5e3f67ce064d913fa515f8a..77ea0cf658fd320267ac74dc65ba9818caa30dc7 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -135,8 +135,6 @@ def normalize_port(options)
           return nil if options[:port].nil? || options[:port] == false
 
           case options[:protocol]
-          when "//"
-            nil
           when "https://"
             options[:port].to_i == 443 ? nil : options[:port]
           else
diff --git a/actionpack/test/controller/url_for_test.rb b/actionpack/test/controller/url_for_test.rb
index a8035e5bd753ead776497875822b501725fef374..0c6df16325a0ebbacccd8569ba952d8037f0d71e 100644
--- a/actionpack/test/controller/url_for_test.rb
+++ b/actionpack/test/controller/url_for_test.rb
@@ -169,6 +169,18 @@ def test_without_protocol
         )
       end
 
+      def test_without_protocol_and_with_port
+        add_host!
+        add_port!
+
+        assert_equal('//www.basecamphq.com:3000/c/a/i',
+          W.new.url_for(:controller => 'c', :action => 'a', :id => 'i', :protocol => '//')
+        )
+        assert_equal('//www.basecamphq.com:3000/c/a/i',
+          W.new.url_for(:controller => 'c', :action => 'a', :id => 'i', :protocol => false)
+        )
+      end
+
       def test_trailing_slash
         add_host!
         options = {:controller => 'foo', :trailing_slash => true, :action => 'bar', :id => '33'}