My error log is showing the following error every five minutes so I suspect it is related to one of my cron jobs,

The error is:

PHP Notice:  Undefined index:  HTTP_USER_AGENT in /home/myaccount/public_html/mysite.com/app/config/config.php on line 18

The relevant line 18 is:

$_is_hashbang_supported_bot = (strpos($_SERVER['HTTP_USER_AGENT'], 'Googlebot') !== false);

Line 19 is

define('IS_HASHBANG_SUPPORTED_BOT', $_is_hashbang_supported_bot);

Any help appreciated.

Many thanks