Adding a note for a subscriber using Mailchimp API - mailchimp

Question

I am having trouble adding a note to a subscriber using Mailchimp's API. Here's what I have:
$email = 'some#email.com';
$list = '123456';
$note = 'This is an example of a note.';
$apiKey = 'MY-API-KEY';
$memberId = md5(strtolower($email));
$dataCenter = substr($apiKey,strpos($apiKey,'-')+1);
$url = 'https://'.$dataCenter.'.api.mailchimp.com/3.0/lists/'.$list.'/members/'.$memberId.'/notes';
$json['note'] = $note;
$json = json_encode($json);
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);
curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 10);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, $json);
$result = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);
if ($httpCode == 200) {
return true;
} else {
return false;
}
Any ideas why these notes would not be saved? I've made sure the API key, the list ID and the email address are all correct.
The documentation page is here: https://developer.mailchimp.com/documentation/mailchimp/reference/lists/members/notes/