**Problem**

I want to break the select list on my exposed form into chunks using select list dividers.

![enter image description here][1]

However this breaks my search. 

Any attempt to search with values from that select list removes all results and attempts to navigate to the next page display the following error:

"An illegal choice has been detected. Please contact the site administrator"

What is going on?

**EDIT --**

Alfred-Armstrong's answer has fixed the first problem. I now get search results. But the form alter is still breaking my ability to change pages. Any more insight would be fantastic.

**Set-Up**

I have an exposed view form. It has a select list field as one of the filter options. 

There are a lot of options in the select list, so I've reordered it (away from alphabetical) to leave the five most popular at the top. This seems a little random so I then altered the form to add in un-selectable dividers. (see top picture)

http://drupal.stackexchange.com/questions/27539/how-to-add-separator-option-in-form-api-select-element (this is for creating forms, I put it into a form alter) 

    function mymodule_form_alter(&$form, $form_state, $form_id) {
        if ($form_id === 'views_exposed_form') {
          'Most Popular' => array (
              1 => 'China',
              2 => 'Thailand',
              3 => 'Japan',
              4 => 'Spain',
              5 => 'Saudi Arabia',
          ),
          
          'Countries' => array (
              6 => 'Afghanistan',
              7 => 'Aland Islands',
              8 => 'Albania',
              9 => 'Algeria',
              10 => 'American Samoa',
              ..
           );
        }

Is it somehow appending "Most Popular" or "Countries" onto the select list data value, invalidating the select list and throwing errors?

Allowed values in my select list looks like:

    China|China
    Thailand|Thailand
    Japan|Japan
    Saudi Arabia|Saudi Arabia
    Spain|Spain
    Afghanistan|Afghanistan
    Aland Islands|Aland Islands
    Albania|Albania
  [1]: https://i.sstatic.net/Deutd.png