Ruby on rails : undefined method 'map' for #<String>

I am trying to create a rails form using and tag, but I want to generate options with a json file, because I want all the countries. But I have this error :

undefined method `map' for #<String:0x007f871472e9b0> 

Here is my application_helper.rb :

module ApplicationHelper
  def countries_for_select
    file = File.read(File.join(Rails.root, 'app', 'helpers', 'countries.json')).to_json
    countries = JSON.parse(file)
    countries.map {|c| [ c['country']['name'], v['country']['code'] ] }
  end
end

Here is my posts_controller.rb :

  def create
    countries_for_select
    @post = Post.new(posts_params)
    @post.user = current_user
    options_for_countries
    if @post.save
      flash[:success] = "Your post have been published"
      redirect_to post_show_path
    else
      render 'new'
    end
  end

Here is the line in my _form.html.erb file :

<%= select_tag(:country, countries_for_select) %>

So I don't understand why it doesn't work, Does someone could help me ?

Thank you !