Error: options: expected array, got string - but it is an array?

Below is my complete code. And here is the result:

Error in call to API function "files/search_v2": options: expected array, got string

$token = "....";
$options = array("path" => "/", "max_results" => 10);
$options["file_categories"] = "folder";
$parameters = array("query" => "searchPhrase", "options" => $options);

$headers = array(
    "Authorization: Bearer $token",
    'Content-Type: application/json'
);

$curlOptions = array(
    CURLOPT_HTTPHEADER => $headers,
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => json_encode($parameters),
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_VERBOSE => true,
    CURLOPT_SSL_VERIFYHOST => false,
    CURLOPT_SSL_VERIFYPEER => false
);

try {

    $ch = curl_init('https://api.dropboxapi.com/2/files/search_v2');
    curl_setopt_array($ch, $curlOptions);

    $response = curl_exec($ch);
    //var_dump($response);
    curl_close($ch);

} catch (Exception $e) {

    trigger_error(
        sprintf(
            'Curl failed with error #%d: %s',
            $e->getCode(),
            $e->getMessage()
        ),
        E_USER_ERROR
    );
}

Some more debugging:

echo json_encode($parameters, JSON_PRETTY_PRINT);

{
    "query": "cat",
    "options": {
        "path": "\/folder",
        "max_results": 10,
        "file_categories": "folder"
    }
}
Error in call to API function "files/search_v2": options: expected array, got string

That sure looks like an array to me, so I don't understand why it says it's a string...