Error HTTP_USER_AGENT

Hallo, from log file this plugin make a few of error:
user agent – PHP Undefined index: HTTP_USER_AGENT on wp-content/plugins/simple-social-buttons/simple-social-buttons.php on line 1622

I fix this
replace this line 1622

$useragent = $_SERVER[‘HTTP_USER_AGENT’];
from simple-social-buttons.php
with this code
$useragent = isset($_SERVER[‘HTTP_USER_AGENT’]);