[Rails] Need help fixing an error populating a select box in the insert page but works in the editpage

Two Models Country and Bank. The below form is used for insert and edit page works fine .  f.collection_select works fine for edit page but throws error for insert page. I think error it is trying to populate the selected value in the combo box but country id is not available. Can you help me fixing the issue? What is prompt used for?

Error

undefined method `map' for nil:NilClass

Extracted source (around line #23):

20:     <%= f.label :country_id %><br />  21:      22:        23:      <%= f.collection_select :country_id, @countries, :id, :name, :prompt => "Test" %>  24:     25:   <div class="actions">  26:     <%= f.submit %>  

class Bank < ActiveRecord::Base

  attr_accessible :name,:country_id

  belongs_to :country

  validates :name, :presence => true

  

    validates_presence_of :country

end

class Country < ActiveRecord::Base

  attr_accessible :name

 end

Form

<%= form_for(@bank) do |f| %>

  <% if @bank.errors.any? %>

    <div id="error_explanation">

      <h2><%= pluralize(@bank.errors.count, "error") %> prohibited this bank from being saved:</h2>

      <ul>

      <% @bank.errors.full_messages.each do |msg| %>

        <li><%= msg %></li>

      <% end %>

      </ul>

    </div>

  <% end %>

  <div class="field">

    <%= f.label :name %><br />

    <%= f.text_field :name %>

  </div>

  

  <div class="field">

    <%= f.label :country_id %><br />

    

     

     <%= f.collection_select :country_id, @countries, :id, :name, :prompt => "Test" %>

 

  

  <div class="actions">

    <%= f.submit %>

  </div>

<% end %>

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