git-fetch will leave a ref pointing to a tag

I did this while trying to force a fast-forward merge (I wanted an error message if I had changes to merge), and got a somewhat obscure error message:

$ git branch temp tags/v2.6.22-rc6 $ git fetch . tags/v2.6.22-rc7:temp $ git checkout temp $ (make minor change) $ git commit -a fatal: 087ea061253277de2b27e82d8572a386835a1b7e is not a valid 'commit' object

git-fetch does odd things when handed a tag rather than a commit. Also, should "git checkout" have complained?