[installer] Undefined variable $issued_successfully

Output from the installer:

Do you want a secure (SSL) connection to the ISPConfig web interface (y,n) [y]: 

Checking / creating certificate for _hostname_
Using certificate path /etc/letsencrypt/live/_hostname_
Server's public ip(s) (_IP_) not found in A/AAAA records for _hostname_: _IP_
Ignore DNS check and continue to request certificate? (y,n) [n]: 

PHP Warning:  Undefined variable $issued_successfully in /tmp/ispconfig3_install/install/lib/installer_base.lib.php on line 3319
Could not issue letsencrypt certificate, falling back to self-signed.