Adding a validation handler to the registration form I have altered the registration form to change the thank you message and add the validation of some fields.

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'user_register_form') {
    $form['actions']['submit']['#validate'][]  = 'checkout_nationality_form_validate';
    $form['#submit'][] = 'thank_on_submit';
  }


All code is working fine; my problem is the registration doesn't take the default Drupal validation. Why does this happen?
You need to change this line 

 $form['actions']['submit']['#validate'][]  = 'checkout_nationality_form_validate';


to 

 $form['#validate'][] = 'checkout_nationality_form_validate';