bzr diff --old/--new does not report and changes with a lightweight checkout of a native repo

I'm using a native repo in ~/src/juju-core. I have a lightweight checkout of it in ~/go/src/launchpad.net/juju-core/ (in my $GOPATH).
In addition I have a symlink in ~/work/juju-core pointing to the second path above.
I have 2 branches: trunk and 041-provisioner-api-calls

What happens:
$ cd ~/work/juju-core
$ bzr diff --old trunk --new 041-provisioner-api-calls state/api
bzr: ERROR: Path(s) are not versioned: 041-provisioner-api-calls trunk

If instead I do this it works:
$ cd ~
$ bzr diff --old ~/src/juju-core/trunk --new ~/src/juju-core/041-provisioner-api-calls/ state/api

Or even:
$ cd ~/src
$ bzr diff --old trunk --new 041-provisioner-api-calls state/api
Also works.

I'd expect bzr diff working in the lightweight checkout just the same as in the native repo it links to.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: bzr 2.6.0~bzr6571-4ubuntu2
ProcVersionSignature: Ubuntu 3.8.0-22.33-generic 3.8.11
Uname: Linux 3.8.0-22-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Fri May 24 12:49:05 2013
InstallationDate: Installed on 2011-11-17 (553 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: bzr
UpgradeStatus: Upgraded to raring on 2013-05-03 (20 days ago)