Anonymous username deleted if someone comment

Using Anspress 4 & WP 4.7.2

If someone commenting on anonymous question/answer, the username get deleted.

“jack (anonymous) Posted 1 minute ago” will become “Anonymous Posted 1 minute ago”

Error recorded:

[06-Mar-2017 11:04:10 UTC] PHP Notice:  Trying to get property of non-object in /var/www/satu.com/wp-content/plugins/anspress-master/addons/free/email.php on line 601