Unable to use ubuntu-bug: This is not an official Linux package

On Linux Mint 19.3/Bionic 18.04 LTS

$ ubuntu-bug systemd results in
*** Problem in systemd
The problem cannot be reported:
This is not an official Linux package. Please remove any third party package and try again.

Expected outcome: file a bug against systemd

$ apt-cache policy systemd
systemd:
  Installed: 237-3ubuntu10.38
  Candidate: 237-3ubuntu10.38
  Version table:
 *** 237-3ubuntu10.38 500
        500 http://mirror.unix-solutions.be/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     237-3ubuntu10 500
        500 http://mirror.unix-solutions.be/ubuntu bionic/main amd64 Packages

$ apt-cache policy apport
apport:
  Installed: 2.20.9-0ubuntu7.9
  Candidate: 2.20.9-0ubuntu7.9
  Version table:
 *** 2.20.9-0ubuntu7.9 500
        500 http://mirror.unix-solutions.be/ubuntu bionic-updates/main amd64 Packages
        500 http://mirror.unix-solutions.be/ubuntu bionic-updates/main i386 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main i386 Packages
        100 /var/lib/dpkg/status
     2.20.9-0ubuntu7 500
        500 http://mirror.unix-solutions.be/ubuntu bionic/main amd64 Packages
        500 http://mirror.unix-solutions.be/ubuntu bionic/main i386 Packages

As recommend by Linux Mint, I enabled of course alternative/mirror sources.
Maybe that is why it thinks it's a third party package.
Of course, I can report the bug manually, so this is just a minor issue, but of course apport is doing it much better...