diff --git a/actionpack/lib/action_dispatch/journey/formatter.rb b/actionpack/lib/action_dispatch/journey/formatter.rb
index 985cf8b9470eb4568c6893aead8deb2d4bad8970..95e1d2deb2309f8f0672d3c3468dc94999cc72cc 100644
--- a/actionpack/lib/action_dispatch/journey/formatter.rb
+++ b/actionpack/lib/action_dispatch/journey/formatter.rb
@@ -32,8 +32,8 @@ def generate(name, options, path_parameters, parameterize = nil)
 
           defaults       = route.defaults
           required_parts = route.required_parts
-          parameterized_parts.delete_if do |key, value|
-            value.to_s == defaults[key].to_s && !required_parts.include?(key)
+          parameterized_parts.keep_if do |key, value|
+            defaults[key].nil? || value.to_s != defaults[key].to_s || required_parts.include?(key)
           end
 
           return [route.format(parameterized_parts), params]