branching with with 0 revisions causes an error

(Steps to reproduce follow) I am currently in the process of evaluating bzr as an existing svk/svn user. As part of my experimentation I initialised a repo, then created a branch and immediately branched it. On the new branch I committed a file and merged the original branch with the new branch (still following?!) The merge appeared to work fine, but when issuing bzr status in the original branch I got the following error:

working tree is out of date, run 'bzr update'

I dutifully ran bzr update, but the following happened:

-D testfile
All changes applied successfully.
bzr: ERROR: Reserved revision-id {null:

I've tried this a few times and can reproduce it each time, and have come to the conclusion that if you create a branch in a repository with 0 revisions, subsequent merges into the original branch cause a strange error in bzr.

I know it is a very unlikely situation, but an error message stating that it's invalid to branch at this revision would be more user friendly.

Steps to reproduce:

mkdir testdir && cd testdir
bzr init-repo test_repo && cd test_repo
bzr init live
bzr branch live branch1
cd branch1
vi testfile # enter some text and save
bzr add testfile
bzr commit -m "new file" testfile
cd ../live
bzr merge ../branch1
#... all seems OK so far .. continue..
bzr status # says working tree out of date
bzr update # strange message appears at this point.

Output from bzr --version:
Bazaar (bzr) 1.5
  Python interpreter: /usr/bin/python 2.5.2
  Python standard library: /usr/lib/python2.5
  bzrlib: /usr/lib/python2.5/site-packages/bzrlib
  Bazaar configuration: /home/pkelly/.bazaar
  Bazaar log file: /home/pkelly/.bzr.log

Using ubuntu hardy heron.