Bug in git remote prune?

Let's say you've got the following tracking branches:

refs/remotes/origin/master
refs/remotes/origin/next
refs/remotes/origin/ghost

And origin has only "master" and "next" (ghost has been removed upstream).

Further, let's say you've got a refspec of:

[remote "origin"]
  fetch = +refs/heads/master:refs/remotes/origin/master

What should "git remote prune origin" do?

In my opinion, it should prune "next" and "ghost".  "ghost" is gone
from upstream, so that's obvious. I think "next" should also be pruned
because the refspec indicates the user is no longer interested in it.

Currently it doesn't do anything (it only considers "master" per the
refspec, which is still on origin, so it doesn't think there is
anything to prune).

Thoughts?

j.