diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 66ab74ee..a43954b5 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -26,8 +26,6 @@ class Profile < Model
 end
 
 class Post < Model
-  attr_writer :comments
-
   def comments
     @comments ||= [Comment.new(content: 'C1'),
                    Comment.new(content: 'C2')]
diff --git a/test/unit/active_model/array_serializer/serialization_test.rb b/test/unit/active_model/array_serializer/serialization_test.rb
index c19b51c3..46ea3fcb 100644
--- a/test/unit/active_model/array_serializer/serialization_test.rb
+++ b/test/unit/active_model/array_serializer/serialization_test.rb
@@ -53,6 +53,10 @@ module ActiveModel
         @post1 = Post.new({ title: 'Title 1', body: 'Body 1', date: '1/1/2000' })
         @post2 = Post.new({ title: 'Title 2', body: 'Body 2', date: '1/1/2000' })
 
+        class << @post2
+          attr_writer :comments
+        end
+
         @post2.comments = [
           Comment.new(content: 'C3'),
           Comment.new(content: 'C4')