mmdebstrap'ping sh4 failure on amd64 host

Hi everyone,

I am investigating how we can extend autopkgtest-virt-qemu's support to
more architectures, and found that creation of a root filesystem of sh4
by mmdebstrap on an Debian testing amd64 host fails.
John told me that I should report it to this list at
https://salsa.debian.org/ci-team/autopkgtest/-/merge_requests/97#note_205337

Specifically,

mmdebstrap --include=debian-ports-archive-keyring --architectures=sh4 sid /tmp/some-dir - <<EOF
deb http://deb.debian.org/debian-ports/ sid main
deb http://deb.debian.org/debian-ports/ unreleased main
EOF

fails with

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 isc-dhcp-client : Depends: libdns-export1104 but it is not installable
                   Depends: libisc-export1100 but it is not installable
E: Unable to correct problems, you have held broken packages.

Best regards, Ryutaroh Matsumoto