Misleading depreciation warning for form_for

I use form_for to access a serialized instance: <%= fields_for :settings, @my_model.settings do |settings_form| %> where "settings" is the serialized instance. This works fine, but I get the below depreciation. The problem is that when using as suggested <%= fields_for @my_model.settings, :as => settings do |settings_form| %> I get some missing model_name error. Of course it has no model name, as it is not an ActiveRecord model. But the depreciated form_for way doesn't have that problem. Is there a better way now to use a serialized field together with form_for?

DEPRECATION WARNING: Using form_for(:name, @resource) is deprecated.

This is talking about form_for, your code above is fields_for ???