2 files changed, 3 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index e8a782ed13..0bcfa5f00d 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -133,7 +133,7 @@ module ActiveRecord
       end
 
       def previous_mutation_tracker
-        @previous_mutation_tracker ||= NullMutationTracker.new
+        @previous_mutation_tracker ||= NullMutationTracker.instance
       end
 
       def cache_changed_attributes
diff --git a/activerecord/lib/active_record/attribute_mutation_tracker.rb b/activerecord/lib/active_record/attribute_mutation_tracker.rb
index ba7348918b..0133b4d0be 100644
--- a/activerecord/lib/active_record/attribute_mutation_tracker.rb
+++ b/activerecord/lib/active_record/attribute_mutation_tracker.rb
@@ -46,6 +46,8 @@ module ActiveRecord
   end
 
   class NullMutationTracker # :nodoc:
+    include Singleton
+
     def changed_values
       {}
     end