diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index ce6c87cd..f1f7a5b8 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -53,7 +53,7 @@ module ActiveModel
           end
         end
 
-      protected
+        private
 
         def find_serializable(object)
           if target_serializer