New branch name with no other changes

I checked out a new workspace, and I want to check it in again unchenged 
with a new branch name  -- the one I want to use to make changes.  I'd 
like to make it clear that the new branch is starting in the same state 
as the current head in the old one.

But when I try to check in, it fails:

hendrik@notlookedfor:~/fanfic/theShadowChronicles-MarkMacKinnon/edited$ 
mtn commit 
--branch=com.pooq.hendrik.othersfanfic.theShadowChronicles.cleanup
mtn: misuse: no changes to commit

Well, yes, there were no changes in any of the files, but the branch was 
different.

Is there some conceptual reason why a branch name change should not be 
enough to do a commit?

Of course I can get on with my work by making a small inessential 
change, but this is inelegant.  I'll do it, though, when time presses. 

-- hendrik