Backup fails to include the contents of /var/lib/asterisk/sounds/en/custom

Although all modules (other than "Music on Hold") are included in the backup options, when running the backup the system, after adding the directory /var/lib/asterisk/sounds/en/custom fails to include its contents.

The relevant portion of the log is below:

Running Backup ID: 6e370d06-f879-4718-83d5-a5931443d022

Transaction: a3701373-8cfe-4711-b762-5f095d3291d8

Starting backup Week

This backup will be stored locally and is subject to maintenance settings Storage Location:

/var/spool/asterisk/backup/Week/20190531-155028-1559310628-15.0.16-1487830852.tar.gz

Working with recordings module

        Exporting Feature Codes from recordings

        Adding directory to tar: /var/lib/asterisk/sounds/en/custom

        Adding module manifest for recordings

Working with ringgroups module