Bug report: "git-merge --ff" should fail if branches have diverged

Git developers, I have a small bug report:

When I specify the "--ff" option to git-merge, I expect it to perform a fast-forward merge or none at all. If the branches have diverged and a fast-forward cannot be done, I expect the command to fail. With git 1.7.6, if "git-merge -ff" cannot fast-forward, it falls back to creating the merge commit I did not want.

Thank you,
-Sean