is ORIG_HEAD allowed to point to a non-existing object?

If I do a "git-rebase -i ..." followed by "git reflog expire ..." and
"git gc ..." then I can end up with a repo which has a ref ORIG_HEAD
which points to a non-existing object.

- Is this intended?
- What's the reason to keep a ref which points to a non-existing object?
- Are there any other refs which are allowed to point to non-existing objects?

Here is how I reproduced the problem:

git init

Initialized empty Git repository in /tmp/yy/.git/

touch a
git add a
git commit -m addA

[master (root-commit) ff82f68] addA
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a

echo 2 >a
git commit -a -m modA

[master f9ae5a7] modA
 1 file changed, 1 insertion(+)

echo 3 >a
git commit -a -m 'fixup! modA'

[master 271012b] fixup! modA
 1 file changed, 1 insertion(+), 1 deletion(-)

git rebase -i --autosquash HEAD~~

[detached HEAD 3eb17d4] modA
 1 file changed, 1 insertion(+)
Successfully rebased and updated refs/heads/master.

git rev-parse --short ORIG_HEAD

271012b

git reflog expire --expire=now --all
git rev-parse 271012b

271012be467fcbd62a875b7187c0e9871f62ac0d

git gc --prune=now

Counting objects: 6, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (6/6), done.
Total 6 (delta 1), reused 0 (delta 0)

git rev-parse 271012b

271012b
fatal: ambiguous argument '271012b': unknown revision or path not in
the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

git rev-parse --short ORIG_HEAD

271012b

Ciao
  Chris