GET /person/ with custom field returning NULL

I’m trying to get values from a custom field(
birth date) from my pipedrive.

1- I made GET/person/ request to see whitch is the index that I needed that refers my custom field (birth date).

2 - After the step 1, I saw that my index was b6c3554a1267b01b16398ed31c61ea9ae13cf6d9 from the custom field and made the following PHP code:

// Pipedrive API token
$api_token = token;

// Pipedrive company domain
$company_domain = company;
//URL for Deal listing with your $company_domain and $api_token variables
$url = 'https://'.$company_domain.'' . $api_token;



                        //b6c3554a1267b01b16398ed31c61ea9ae13cf6d9 <--- birth date
                        $pag = 0;
                        $limit = 100;
                        $more_items= true;
                        while($more_items ==true){
                        $data = http_build_query(array('start'=>($pag*$limit),'limit'=>$limit,'filter_id'=>69));
                        //GET request
                        $ch = curl_init();
                        curl_setopt($ch, CURLOPT_URL, $url."&".$data);
                        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
                        $output = curl_exec($ch);
                        // Create an array from the data that is sent back from the API
                        // As the original content from server is in JSON format, you need to convert it to a PHP array
                        $result = json_decode($output, true);
                            $persons = $result["data"];
                            foreach($persons as $person){
                                $more_items = true;
                            $more_items = false;

   catch( Exception $e ) {
    echo json_encode($e->getMessage());


3 - I expected to receive all the names and birth dates, but I’m receiving some birth days and a lot of NULL’s instead.

4- prints:

Hello @simili_musica

Could you run GET /persons/<personId> (details), where personId any person ID where birthday custom field was NULL? After that, could you check if birthday data is actually present under that custom field?

Thank you for replying @mykhailo.
I runned GET /persons/<personId> and realised that b6c3554a1267b01b16398ed31c61ea9ae13cf6d9 is NULL, but another index is filled with the birth date data, I was using an wrong index.
After using the right index I could get the correctly birth date data.