Appending value to DynamoDB number set using aws-php-sdk - php

Question

I am using aws-php-sdk with dynamoDB to store a set of unique numerical values for a key.
I want to be able to append a new value to the set - this is easy to achieve with a list however this also results in duplicate values so I would rather use a set.
Because of the way dynamoDB is charged I don’t want to have to read all the values, insert unique value and write the whole set back because that would be very costly when scaled out. Docs seem to suggest appending to a number set is possible but doesn’t give much help in making it work.
$UpdateExpression = "
SET #subscribers = list_append(if_not_exists(#subscribers, :empty_list), :subscriber)
";
// This condition should match if the value already exists.
$ConditionExpression = "not contains (#subscribers,:subscriberStr)";
$ExpressionAttributeNames = [
"#subscribers" => "subscribers"
];
$ExpressionAttributeValues = $marshaler->marshalJson('
{
":subscriber": ['.$subscriber.'],
":empty_list": [],
":subscriberStr": "'.$subscriber.'"
}
');
$tableName = 'publisherRelationships';
$key = $marshaler->marshalJson('
{
"publisher": '.$publisher.'
}
');
$params = [
'TableName' => $tableName,
'Key' => $key,
'UpdateExpression' => $UpdateExpression,
'ConditionExpression' => $ConditionExpression,
'ExpressionAttributeNames' => $ExpressionAttributeNames,
'ExpressionAttributeValues' => $ExpressionAttributeValues
];
$result = $client->updateItem($params);