Why Minitest (Rails 4.2) failed when I add a key/value column (Hstore) in a fixture definition?

Question

I'm using rails 4.2, minitest 5.11.3, and minitest-rails 2.2.1

My test class has some problem when I want to exploit hstore field from postgresql into fixture.

Here my test class WelcomeMailerTest when running test is ok:

require 'test_helper'
class WelcomeMailerTest < ActionMailer::TestCase

  describe 'user_mail_test' do
    let(:user) {users(:anthony)}
    let(:email) {WelcomeMailer.user(user.id)}

    def test_email_to
        assert_equal [user.email], email.to
    end
  end
end

My fixture user in the users.yml file:

anthony:
  username: anthony-mattiocco 
  first_name: Anthony
  last_name: Mattiocco 
  email: [email protected]
  token: token
  slug: antho

When I use a new fixture coutries.yml with key/value field (hstore), the test failed.

Here my new fixture coutries.yml :

us:
  alpha2: US 
  slugs: <%= {:slug_en => 'test'}.to_yaml.inspect %>
  nationality: 'American'
  currency: 'USD'

and my updated WelcomeMailerTest class :

require 'test_helper'

class WelcomeMailerTest < ActionMailer::TestCase
  describe 'user_mail_test' do
    let(:country) {countries(:us)}
    puts("countries : " + country.inspect)
    let(:user) {users(:anthony)}
    let(:email) {WelcomeMailer.user(user.id)}

    def test_email_to
        assert_equal [user.email], email.to
    end
  end
end

When I execute :

rake test

I got the following error :

Error: user_mail_test#test_email_to: ActiveRecord::StatementInvalid: PG::InternalError: ERROR: Syntax error near ':' at position 4 LINE 1: ..., "currency", "created_at", "updated_at") VALUES ('US', '--- ^ : INSERT INTO "countries" ("alpha2", "slugs", "nationality", "currency", "created_at", "updated_at") VALUES ('US', '--- :slug_en: test ', 'American', 'USD', '2019-11-14 23:02:06', '2019-11-14 23:02:06')

Thanks in advance for your help