Quoting Dragan FOSS (dragan.foss@???):

> >Oops!
> Actually (and TBH..), this is a bug in libidn2, which isn't in
> systemd source tree...at least for now ;>

It turns out to be a GNU library: 'Libidn's purpose is to encode and
decode internationalized domain names.' That's IDN = internationali[sz]ed
domain name.

https://www.gnu.org/software/libidn/