diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 800c6b469ebce..00f8fb99bb971 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -774,7 +774,7 @@ def apply_common_behavior_for(method, resources, options, &block)
               return true
             end
 
-            options.each do |k,v|
+            options.keys.each do |k|
               (options[:constraints] ||= {})[k] = options.delete(k) if options[k].is_a?(Regexp)
             end