diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index ddd93e34a6..d2ebd18107 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -257,7 +257,7 @@ def mutations_from_database
         unless defined?(@mutations_from_database)
           @mutations_from_database = nil
         end
-        @mutations_from_database ||= if @attributes
+        @mutations_from_database ||= if defined?(@attributes)
           ActiveModel::AttributeMutationTracker.new(@attributes)
         else
           NullMutationTracker.instance
@@ -265,7 +265,7 @@ def mutations_from_database
       end
 
       def forget_attribute_assignments
-        @attributes = @attributes.map(&:forgetting_assignment) if @attributes
+        @attributes = @attributes.map(&:forgetting_assignment) if defined?(@attributes)
       end
 
       def mutations_before_last_save