Bug? git worktree fails with master on bare repo

Hey all, I'm working on some server-side software to do a merge. By using git worktree it's possible to check out a given branch for a bare repo and merge another branch into it. It's very fast, even with large repositories.

The only exception seems to be merging to master. When I do git worktree add /tmp/path/to/worktree master I get an error:

[fatal: 'master' is already checked out at '/path/to/bare/repo']

But this is clearly not true, git worktree list gives:

[/path/to/bare/repo (bare)]

...and of course, there is no work tree at that path, just the bare repo files you'd expect.