svn: "svn up" fails if git repo has submodules

If a git submodule is present (there is a .gitmodules file) in a git repository, svn access to that repository works fine for initial checkout, but "svn up" reports it has fetched the new commit and... nothin'. It's present in the .svn folder but none of the new changes show up in the working directory.

If I remove the git submodule from the git repository and try again... everything works great.