active_record-acts_as nested form

Question

I have integrated active_record-acts_as gem for role based attributes management. I have user as the parent model and doctor, student as the acts_as model as below,

class User < ActiveRecord::Base
  actable
end

class Doctor < ActiveRecord::Base
  acts_as :user
end

class Student < ActiveRecord::Base
  acts_as :user
end

some common fields like first_name, last_name, contact_no belongs to user and uncommon fields goes to other respective models. I have integrated step form using wicked gem. Now when user signs up he is redirected to the second step where I want to update few fields from user and few fields from doctor model.

How can I use nested form ??

my form object belongs to doctor model

<h3> Let us know more about yourself </h3>
 
<%= render layout: 'form' do |f| %>
  <div class="field">
    <%= f.label :existing_email %><br />
    <%= f.text_field :existing_email %>
  </div>

  <%= f.fields_for f.object.acting_as do |ff| %>
    <div class="field">
      <%= f.label :mobile_no %><br />
      <%= ff.text_field :mobile_no %>
    </div>  

    <div class="field">
      <%= f.label :country_id %><br />
      <%= ff.text_field :country_id %>
    </div>  
  <% end %>
<% end %>

controller for wicked#steps management

class RegistrationStepsController < ApplicationController
  include Wicked::Wizard
  
  steps :step_two
  
  def show
    @user = current_user.specific #docotr object
    render_wizard
  end
  
  def update
    @user = current_user
    @user.specific.attributes = doctor_params
    render_wizard @user
  end
  
    private

  def doctor_params
    params.require(:doctor).permit!
  end

  def redirect_to_finish_wizard
    redirect_to root_url, notice: "Thank you for signing up."
  end
end

getting error as,

User(#154180) expected, got {"mobile_no"=>"+919702610970", "country_id"=>"India"} which is an instance of ActiveSupport::HashWithIndifferentAccess(#54060)