Invalid large, negative Content-Length

When trying to "svn update" a working copy on Linux (svn 1.6.6) I'm receiving
the error "413 Request Entity Too Large".

Indeed, the Content-Length I'm seeing the client send when I set
neon-debug-mask = 138 in [global] of the servers file is:
Content-Length: -5221747158880878327

This seems similar to bug #3558 [1] but I'm just trying to update a working
repository, I don't have any 2+GB files, and I'm on Linux and not Windows.

Any ideas on what I can do to debug this?

-Doug

[1] http://subversion.tigris.org/issues/show_bug.cgi?id=3558