public function getBalance($email, $appUid = null)
    {
        if (null === $appUid) $appUid = $this->defaultAppUid;
        if (!$appUid) throw new \InvalidArgumentException('No appUid specified');

        $response = $this->apiClient->get('app_store/balance', [
            'app_uid' => $appUid,
            'email' => $email,
        ]);

        if (!isset($response['num_credits'])) throw new \ErrorException('Unexpected API response');
        return $response['num_credits'];
    }