Reverting a subdirectory of a repo

Recently I wanted to revert a subdirectory of a repo to an earlier state, so I did this:

$ git checkout <revision> -- /path/to/subdir $ git commit

And I was surprised that the subdir retained files that had been added since <revision>. To get a clean revert, I had to:

$ git rm -r /path/to/subdir $ git checkout <revision> -- /path/to/subdir $ git commmit

And I'm wondering if requiring the "rm" first is bug or a feature. On the one hand, in retrospect I understand why it's needed, but otoh, it did catch me off guard the first time.

j.