diff --git a/lib/rack/cors/resource.rb b/lib/rack/cors/resource.rb
index aa6ae35..2fb1daa 100644
--- a/lib/rack/cors/resource.rb
+++ b/lib/rack/cors/resource.rb
@@ -66,7 +66,7 @@ def to_headers(env)
           'access-control-max-age' => max_age.to_s
         }
         h['access-control-allow-credentials'] = 'true' if credentials
-        defined?(Rack::Utils::HeaderHash) ? Rack::Utils::HeaderHash.new(h) : h
+        header_proc.call(h)
       end
 
       protected
@@ -127,6 +127,16 @@ def compile(path)
           raise TypeError, path
         end
       end
+
+      def header_proc
+        @header_proc ||= begin
+          if defined?(Rack::Headers)
+            ->(h) { h }
+          else
+            ->(h) { Rack::Utils::HeaderHash.new(h) }
+          end
+        end
+      end
     end
   end
 end