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
<%= 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

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

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