bzr update of revisionless master falsely claims succes

On irc a user (Tak) reported a problem where bzr told him to run update when he tried to commit, he did so, bzr told him all was fine, he committed again and got the exact same message. Message in question:

  bzr: ERROR: Bound branch BzrBranch6('file:///home/larstiq/tmp/ex/slave/') is out
   of date with master branch BzrBranch6('file:///home/larstiq/tmp/ex/master/').
  To commit to master branch, run update and then commit.
  You can also pass --local to commit to continue working disconnected.

Steps to reproduce:

bzr init master
bzr co master slave
cd slave
touch foo
bzr add foo
bzr ci --local -m 1
bzr ci # get message
bzr up # updated to revision 1
bzr ci # get message again