svn diff refuses to pass no arguments to external diff program

I like to use meld as an external diff program which svn diff.

For instance, I used to be able to run
    svn diff --diff-cmd meld
and everything would just work.

Now I get the following error:

ed@ed-Ubu64:~/f11/trunk$ svn diff --diff-cmd meld
Index: ocaml/traces.ml
===================================================================
Xlib: extension "RANDR" missing on display ":0.0".
Usage:
  meld Start with an empty window
  meld <file|dir> Start a version control comparison
  meld <file> <file> [<file>] Start a 2- or 3-way file comparison
  meld <dir> <dir> [<dir>] Start a 2- or 3-way directory comparison
  meld <file> <dir> Start a comparison between file and dir/file

meld: error: no such option: -u
svn: 'meld' returned 2

svn is passing -u to meld. That's okay, this is documented. From svn help:

  -x [--extensions] ARG : Default: '-u'. When Subversion is invoking an
                             external diff program, ARG is simply passed along
                             to the program. But when Subversion is using its
                             default internal diff implementation, or when
                             Subversion is displaying blame annotations, ARG
                             could be any of the following:

So, if I run svn diff --diff-cmd meld -x "" it should not pass -u to meld. Let's try it:

Index: ocaml/traces.ml
===================================================================
Xlib: extension "RANDR" missing on display ":0.0".
Usage:
  meld Start with an empty window
  meld <file|dir> Start a version control comparison
  meld <file> <file> [<file>] Start a 2- or 3-way file comparison
  meld <dir> <dir> [<dir>] Start a 2- or 3-way directory comparison
  meld <file> <dir> Start a comparison between file and dir/file

meld: error: no such option: -u
svn: 'meld' returned 2

Nope, still passes -u. Using a non-empty string works:
    svn diff -x "-L hello" --diff-cmd meld

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: subversion 1.6.12dfsg-4ubuntu2.1
ProcVersionSignature: Ubuntu 2.6.38-11.50-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic x86_64
NonfreeKernelModules: nvidia wl
Architecture: amd64
Date: Thu Sep 29 16:30:04 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: subversion
UpgradeStatus: No upgrade log present (probably fresh install)