Repairing fatal: ref HEAD is not a symbolic ref (git checkout of svn remote)

To test a particular upstream revision, on a git svn remote checkout (i.e. branches/1.2, 76c7af2...), I checked out the equivalent of HEAD^^ (git checkout 4a3d99c0c9).

Now back at the branch tip, I get fatal: ref HEAD is not a symbolic ref on git svn rebase:

(4a3d99c...) $ git checkout 76c7af2 HEAD is now at 76c7af2... Minor changes to CONTRIBUTORS (76c7af2...) $ git svn rebase fatal: ref HEAD is not a symbolic ref Current branch HEAD is up to date. Segmentation fault

How can I manually correct ref HEAD?

Thanks.

(The segmentation fault is a separate issue)