dirty_attribute.rb

diff --git i/lib/virtus/dirty_attribute.rb w/lib/virtus/dirty_attribute.rb
index 3854a36..13c20a3 100644
--- i/lib/virtus/dirty_attribute.rb
+++ w/lib/virtus/dirty_attribute.rb
@@ -16,7 +16,7 @@ module Virtus
       def _create_writer_with_dirty_tracking(name)
         class_eval <<-RUBY, __FILE__, __LINE__ + 1
           def #{name}=(new_regular_object)
-            prev_object = #{name}
+            prev_object = instance_variable_get(:@#{name})
             new_virtus_object = super
 
             if attribute_dirty?(:#{name}) && original_attributes[:#{name}] == new_virtus_object
diff --git i/lib/virtus/dirty_attribute/session.rb w/lib/virtus/dirty_attribute/session.rb
index 29798ad..d00d1ae 100644
--- i/lib/virtus/dirty_attribute/session.rb
+++ w/lib/virtus/dirty_attribute/session.rb
@@ -8,7 +8,7 @@ module Virtus
       end
 
       def original_attributes
-        @_original_attributes ||= subject.attributes.dup
+        @_original_attributes ||= get_original_attributes(subject).dup
       end
 
       def dirty_attributes
@@ -32,6 +32,15 @@ module Virtus
         original_attributes.clear
         dirty_attributes.clear
       end
+
+      private
+
+      def get_original_attributes(subject)
+        subject.class.attribute_set.each_with_object({}) do |attribute, attributes|
+          name = attribute.name
+          attributes[name] = subject.instance_variable_get("@#{name}")
+        end
+      end
     end
   end
 end