Undefined variable sitename on line 140

Hello,

Nice plugin. It works, the emails get sent, however I get error Undefined variable sitename on line 140 of wp-mail.php. The email gets sent.
I got this error using the BackupWordpress plugin which sends email once the backup is ready for download.

All best,
Goran

http://wordpress.org/extend/plugins/mailgun/