diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index 085c60a..30c632c 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -491,7 +491,7 @@ def each_capitalized
   alias canonical_each each_capitalized
 
   def capitalize(name)
-    name.to_s.split('-').map {|s| s.capitalize }.join('-')
+    name.to_s.split('-'.freeze).map {|s| s.capitalize }.join('-'.freeze)
   end
   private :capitalize