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?