dirname errors printed to console on startup

I have byobu by default enabled (20110824 cloud image).

When I log in, I see some dirname errors flash by. They're reproducible via 'su', so heres the captured output:

$ sudo su -
dirname: missing operand
Try `dirname --help' for more information.
dirname: missing operand
Try `dirname --help' for more information.
To launch in a nested session, run: byobu

Commenting out the followoing stanza in /usr/lib/byobu/.dirs will clean them up.
elif [ -d "$(dirname $TMUX)" ]; then
        # Or perhaps try the tmux tmpdir
        export BYOBU_RUN_DIR="$(dirname $TMUX)/$PKG"
else

You can avoid a 'dirname' fork ( you have 2 of them) there completely with:
elif [ -d "${TMUX%/*}" ]; then
   # Or perhaps try the tmux tmpdir
   export BYOBU_RUN_DIR="${TMUX%/*}/$PKG"
else

that will work for any valid file path that has a '/' in it (ie, it wont work for '.').

You can at very least save 1 fork with:

elif dir=$(dirname "$TMUX") && [ -d "$TMUX" ] ; then
   export BYOBU_RUN_DIR="$dir/$PKG"
fi

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: byobu 4.30-0ubuntu1
ProcVersionSignature: User Name 3.0.0-9.14-virtual 3.0.3
Uname: Linux 3.0.0-9-virtual x86_64
Architecture: amd64
Date: Thu Aug 25 13:44:40 2011
Ec2AMI: ami-0000004b
Ec2AMIManifest: FIXME
Ec2AvailabilityZone: nova
Ec2InstanceType: <nova.db.sqlalchemy.models.InstanceTypes object at 0x4d71150>
Ec2Kernel: aki-00000026
Ec2Ramdisk: ari-00000028
PackageArchitecture: all
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ScreenRC:

SourcePackage: byobu
UpgradeStatus: No upgrade log present (probably fresh install)