git-svn "cannot lock ref" error during fetch

Hi all,

I'm new to git, and I'm experimenting with using git-svn to interact  
with a large SVN repository with lots of branches.

I initially did an init like this:

git-svn init -t tags -b branches -T trunk svn+ssh://server/svn/project

Then I did a git-svn fetch, which started pulling all the branches.  
After a while, however, it hit a branch that it couldn't pull:

Found branch parent: (tags/project-92~9)  
767f1f1601a4deae459c99ea6c1d1b9ba8f57a65
Following parent with do_update
...
Successfully followed parent
fatal: refs/remotes/tags/project-92~9: cannot lock the ref
update-ref -m r13726 refs/remotes/tags/project-92~9  
950638ff72acc278156a0d55baafbabb43f2b772: command returned error: 128

Some amount of searching failed to turn up any hints on what this  
error means or how I can work around it. I'd appreciate any advice ...

James