Purge of pgcluu package leads to an error

vagrant@sid64-vagrant:~/debianize$ sudo apt purge pgcluu
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages were automatically installed and are no longer required:
  javascript-common libjs-jquery libjs-underscore
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  pgcluu*
0 upgraded, 0 newly installed, 1 to remove and 60 not upgraded.
After this operation, 2466 kB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 57388 files and directories currently installed.)
Removing pgcluu (3.1-2) ...
Conf pgcluu disabled.
apache2_invoke prerm: Disable configuration pgcluu
Processing triggers for man-db (2.9.4-2) ...
(Reading database ... 57339 files and directories currently installed.)
Purging configuration files for pgcluu (3.1-2) ...
/var/lib/dpkg/info/pgcluu.postrm: 7: /etc/apache2/envvars: APACHE_CONFDIR: parameter not set
dpkg: error processing package pgcluu (--purge):
 installed pgcluu package post-removal script subprocess returned error exit status 2
Errors were encountered while processing:
 pgcluu
E: Sub-process /usr/bin/dpkg returned an error code (1)

The package does seem to be uninstalled, though.

There is no error with apt remove, and the pakage is correctly uninstalled.

I'm investigating.