Google API Client Libraries -PHP : Google Game Services occurs error (code : 500) without any message - api

Question

I'm using Google API Client Libraries - PHP and using Service accounts issuing on Google API Console.
I want to get Leaderboards list but error occurs code 500 without any message.
Enabled Google Play Game Management, Google Play Game Services, Google+ API on API Console.
And, Android application using this Leaderboards proper.
What can I do resolve this problem?
request codes(PHP)
set_include_path(get_include_path().PATH_SEPARATOR.'/path/to/google-api-php-client/src');
require_once realpath(dirname(__FILE__).'/google-api-php-client/src/Google/autoload.php');
$client_email = 'myaccount#developer.gserviceaccount.com';
$private_key = file_get_contents('/path/to/my_key_file.p12');
$scopes = array(
Google_Service_Games::GAMES,
Google_Service_Games::PLUS_LOGIN
);
$client = new Google_Client();
$credentials = new Google_Auth_AssertionCredentials(
$client_email,
$scopes,
$private_key
);
$client->setAssertionCredentials($credentials);
if ($client->getAuth()->isAccessTokenExpired()) {
$client->getAuth()->refreshTokenWithAssertion();
}
$service = new Google_Service_Games($client);
$response = $service->leaderboards->listLeaderboards();
echo json_encode($response) . "\n";
error occours
Fatal error: Uncaught exception 'Google_Service_Exception' with message 'Error calling GET https://www.googleapis.com/games/v1/leaderboards: (500) {
"error": {
"code": 500,
"message": null
}
}
Additional, It is works simple query - like Book API, returned proper results without error...
$service = new Google_Service_Books($client);
$optParams = array('filter' => 'free-ebooks');
$results = $service->volumes->listVolumes('Korea', $optParams);
var_dump($results) ;
Let me know your any advice.
Thank you for your help. :)