diff --git a/lib/webmock/util/headers.rb b/lib/webmock/util/headers.rb
index e858c1e4..6ab0b011 100644
--- a/lib/webmock/util/headers.rb
+++ b/lib/webmock/util/headers.rb
@@ -6,9 +6,9 @@ module Util
 
     class Headers
 
-      STANDARD_HEADER_DELIMITER = '-'.freeze
-      NONSTANDARD_HEADER_DELIMITER = '_'.freeze
-      JOIN = ', '.freeze
+      STANDARD_HEADER_DELIMITER = '-'
+      NONSTANDARD_HEADER_DELIMITER = '_'
+      JOIN = ', '
 
       def self.normalize_headers(headers)
         return nil unless headers