diff --git a/lib/rack/utils.rb b/lib/rack/utils.rb
index 884e604..341bf2b 100644
--- a/lib/rack/utils.rb
+++ b/lib/rack/utils.rb
@@ -276,7 +276,13 @@ module Rack
       end
 
       def [](k)
-        super(@names[k] ||= @names[k.downcase])
+        super(
+          if @names.key?(k)
+            @names[k]
+          elsif @names.key?(k_downcase = k.downcase)
+            @names[k] = @names[k_downcase]
+          end
+        )
       end
 
       def []=(k, v)