get_avatar() not work

I have custom apps need to get wordpress user data. Here is the code:

public function authUser(ConnectionInterface $conn, array $cookies) {
    global $current_user;
    $current_user = null;
    $wpDir = $conn->Config['wordpress']['dir'];

    $this->debug("WP Autorization cookies:"); $this->debug($cookies);

    foreach ($cookies as $key => $value) {
        $_COOKIE[$key] = urldecode($value);
    }
    require_once($wpDir . '/wp-config.php');
    require_once($wpDir . '/wp-includes/wp-db.php');
    require_once($wpDir . '/wp-includes/pluggable.php');
    $userInfo  = wp_get_current_user();
    if(!$userInfo || !$userInfo->ID){
        return null;
    }

    // get avatar from WordPress
    $avatar = get_avatar( $userInfo->ID, 50 );

    return array(
        'id'            => $userInfo->ID,
        'name'          => $userInfo->display_name,
        'image'         => $avatar
    );
}

‘id’ and ‘name’ is working good… I got the ID and display name. But the ‘image’ is not work getting avatar image. Is there anything I missed here? Any help and suggestion really appreciated. Thank you.