Error on fast-export

Hi!

I'm getting the following error:

16:14:21 Starting export of 1355 revisions ...
brz: ERROR: KeyError: b'null:'

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/breezy/commands.py", line 1036, in exception_to_return_code
    return the_callable(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/breezy/commands.py", line 1228, in run_bzr
    ret = run(*run_argv)
          ^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/breezy/commands.py", line 779, in run_argv_aliases
    return self.run(**all_cmd_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/breezy/commands.py", line 804, in run
    return class_run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/breezy/plugins/fastimport/cmds.py", line 505, in run
    return exporter.run()
           ^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/breezy/plugins/fastimport/exporter.py", line 267, in run
    self.emit_commits(interesting)
  File "/usr/lib/python3/dist-packages/breezy/plugins/fastimport/exporter.py", line 256, in emit_commits
    self.emit_commit(revobj, self.ref, trees[parent], trees[revid])
                                       ~~~~~^^^^^^^^
KeyError: b'null:'

brz 3.3.2 on python 3.11.2 (Linux-6.3.5-2-MANJARO-x86_64-with-glibc2.36)
arguments: ['bzr', 'fast-export', '--marks=./import/marks.bzr', '--git-
    branch=toykeeper', '../upstream/anduril2.tk']
plugins: bash_completion[3.3.2], changelog_merge[3.3.2],
    commitfromnews[3.3.2], cvs[3.3.2], darcs[3.3.2], email[3.3.2],
    fastimport[3.3.2], flake8[unknown], fossil[3.3.2], github[3.3.2],
    gitlab[3.3.2], hg[3.3.2], launchpad[3.3.2], mtn[3.3.2],
    netrc_credential_store[3.3.2], news_merge[3.3.2], po_merge[3.3.2],
    propose[3.3.2], pypi[3.3.2], quilt[unknown], repodebug[3.3.2],
    rewrite[unknown], stats[3.3.2], svn[3.3.2], upload[3.3.2],
    weave_fmt[3.3.2], webdav[3.3.2], zsh_completion[3.3.2]
encoding: 'ascii', fsenc: 'ascii', lang: None

*** Breezy has encountered an internal error. This probably indicates a
    bug in Breezy. You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/brz/+filebug
    including this traceback and a description of the problem.

The script that generates this is:
====================
#!/bin/bash
docker run -it --rm -v `pwd`/update:/root/update -w /root/update debian:bookworm bash -c "\
rm -rf upstream && \
rm -rf anduril2 && \
apt update && \
apt -y upgrade && \
apt -y install bzr-fastimport git && \
git clone https://github.com/void-spark/anduril2.git && \
mkdir upstream && \
cd upstream && \
bzr branch lp:~toykeeper/flashlight-firmware/anduril2 && \
mv anduril2 anduril2.tk && \
bzr branch lp:~gabe/flashlight-firmware/anduril2 && \
mv anduril2 anduril2.gabe && \
cd .. && \
cd anduril2 && \
git remote set-url origin <email address hidden>:void-spark/anduril2.git && \
bzr fast-export --marks=./import/marks.bzr --git-branch=toykeeper ../upstream/anduril2.tk | \
git fast-import --import-marks=./import/marks.git --export-marks=./import/marks.git && \
bzr fast-export --marks=./import/marks.bzr --git-branch=gabe ../upstream/anduril2.gabe | \
git fast-import --import-marks=./import/marks.git --export-marks=./import/marks.git && \
sort ./import/marks.bzr -n -k 1.2 -o ./import/marks.bzr"
===================

I'm not sure how to reproduce it in a smaller example, I hope the stack trace gives enough information