`fatal: You are on a branch yet to be born` while merging GitHub wiki

It took me 5 days to grok surgeon, counting from the first issue I opened. =)

I almost managed to move one repo into subdir of another.

git clone https://github.com/simons-public/protonfixes.git
git clone https://github.com/simons-public/protonfixes.wiki.git
cat <<EOF > MERGESCRIPT
read protonfixes
lint
stats
read protonfixes.wiki
lint
stats
unite protonfixes protonfixes.wiki
rebuild __fixed
EOF
~/p/reposurgeon/reposurgeon "script MERGESCRIPT"

Unfortunately, reposurgeon crashes with fatal: You are on a branch yet to be born.

$ ~/p/reposurgeon/reposurgeon "script MERGESCRIPT"
reposurgeon: These 1 timestamps have multiple commits: 2018-10-15T22:12:06Z
reposurgeon: All commit stamps in this repository are unique.
protonfixes: 2193K, 801 events, 507 blobs, 290 commits, 0 tags, 2 resets, 2020-12-27T20:47:35Z.
  Loaded from protonfixes
reposurgeon: All commit times in this repository are unique.
protonfixes.wiki: 49K, 56 events, 25 blobs, 27 commits, 0 tags, 2 resets, 2020-12-27T20:47:35Z.
  Loaded from protonfixes.wiki
fatal: You are on a branch yet to be born
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x100 pc=0x836388]