The following error makes a regular appearance in logs [Fri

The following error makes a regular appearance in logs

[Fri Jan 15 18:37:15 2016] [warn] [client 157.55.39.25] mod_fcgid: stderr: PHP Notice: Undefined index: function in /var/www/vhosts/MYDOMAIN/httpdocs/wp-content/plugins/wordpress-chat/wpmudev-chat-ajax.php on line 24

Line 24 of the file noted in the error is as follows
if ( $_POST['function'] !== "chat_messages_update" ) {

IP 157.55.39.25 is Microsoft bingbot

Please advise