"Cannot read property 'sha1' of undefined" when tweaking orphan commit

[mo]

1. create a review that contains the initial (orphan) commit for a git repo
2. click the interactive rebase button (because you want to fix a typo in the commit msg)
3. fix the typo in the commit msg
4. click "perform rebase" button

Actual:
Failed to perform rebase.
Server reply: Cannot read property 'sha1' of undefined

Expected:
typo fixed