Apparent bug in git rebase with a merge commit

If you have a git tree and you merge in another, independent git tree so that they are the same, using a merge strategy like this:

$ git merge importing/master -s recursive -Xours

And if you later on want to rebase this merge commit on a newer upstream for whatever reason, you get something like this:

$ git rebase -s recursive -Xours First, rewinding head to replay your work on top of it... fatal: Could not parse object 'ca59931ee67fc01b4db4278600d3d92aece898f4^' Unknown exit code (128) from command: git-merge-recursive ca59931ee67fc01b4db4278600d3d92aece898f4^ -- HEAD ca59931ee67fc01b4db4278600d3d92aece898f4

The reason this occurs is that the first commit of the newly-merged-in code obviously has no parent, so I guess the search for the common ancestor is going to be doomed to fail.

It is possible that I'm misunderstanding the recursive merge strategy; however if this were the case I'd still expect a human-readable error message explaining my mistake rather than a 128 exit code.

For a workaround I'll just re-create the commit, but I thought I'd report this behavior anyway.