partial commit blows up lookin up dir sometimes

Partial commit can't find a parent directory inventory entry sometime. Following script blows up with both bzr.dev and 0.14, when DIR=z; uncommenting DIR=b makes it work just fine.

#!/bin/sh
bzr="bzr"

# This blows up
DIR="z"
# This doesn't
#DIR="b"

rm -rf t
mkdir t
# This happens with --format=knit as well as dirstate
${bzr} init t

cd t
mkdir a
mkdir a/${DIR}
mkdir a/c
echo "x" > a/${DIR}/x
echo "x" > a/${DIR}/y

${bzr} add
${bzr} ci -m'setup'

mkdir a/c/d
${bzr} add a/c/d
${bzr} mv a/${DIR}/x a/c/d/
echo "y" >> a/${DIR}/y
${bzr} ci -m'test' a/${DIR}/y