diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index c4decfbf73..c8de310b9c 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -438,7 +438,7 @@ module ActiveRecord
 
     class ClassCache
       def initialize(class_names, config)
-        @class_names = class_names.dup.stringify_keys
+        @class_names = class_names.stringify_keys
         @config      = config
       end