MongoDuplicateKeyException from MongoClient

test.php

<?php
$client = new MongoClient();
$client->selectDB('test')->drop();
$collection = $client->selectCollection('test', 'image');
$collection->createIndex(
    ['user' => 1, 'imageIdentifier' => 1],
    ['unique' => true]
);
$collection->insert([
    'user' => 'christer',
    'imageIdentifier' => 'image1',
]);

try {
    $collection->insert([
        'user' => 'christer',
        'imageIdentifier' => 'image1',
    ]);
} catch (MongoDuplicateKeyException $e) {
    var_dump($e->getMessage()); // string(167) "localhost:27017: insertDocument :: caused by :: 11000 E11000 duplicate key error index: test.image.$user_1_imageIdentifier_1  dup key: { : "christer", : "image1" }"
}