bzr clean-tree is skipping an unknown directory

[Colin D Bennett]

I ran ‘bzr clean-tree’ when I had several unknown and unwanted
files in my working tree.  It deleted all of them except one.  Now
this one still appears as ‘unknown’ but clean-tree is not
identifying it or trying to remove it:

  $ bzr st
  unknown:
    battmgr.adb
    build_r11/
    build_r11-small/
    build_r11-small-stack-auto/
    init.c.orig
    old_cc2530_regs.h
    old_compiler_defs.h
    build-1.3/src1/
    build-1.3/src2/
  1 shelf exists. See "bzr shelve --list" for details.
  $ bzr clean-tree
  battmgr.adb
  build_r11
  build_r11-small
  build_r11-small-stack-auto
  init.c.orig
  old_cc2530_regs.h
  old_compiler_defs.h
  build-1.3/src2
  Are you sure you wish to delete these [y/N]?y
  deleting paths:
    battmgr.adb
    init.c.orig
    old_cc2530_regs.h
    old_compiler_defs.h
  $ bzr st
  unknown:
    build-1.3/src1/
  1 shelf exists. See "bzr shelve --list" for details.
  $ bzr clean-tree
  Nothing to delete.
  $ bzr st
  unknown:
    build-1.3/src1/
  1 shelf exists. See "bzr shelve --list" for details.
  $ bzr clean-tree --unknown
  Nothing to delete.
  $ bzr st
  unknown:
    build-1.3/src1/
  1 shelf exists. See "bzr shelve --list" for details.
  $ bzr unknowns
  build-1.3/src1
  $ 
  $ bzr version
  Bazaar (bzr) 2.4.1
    Python interpreter: /usr/bin/python 2.7.2
    Python standard library: /usr/lib/python2.7
    Platform: Linux-3.0.0-15-generic-x86_64-with-Ubuntu-11.10-oneiric
    bzrlib: /usr/lib/python2.7/dist-packages/bzrlib
    Bazaar configuration: /home/cdb/.bazaar
    Bazaar log file: /home/cdb/.bzr.log
  ...

What's going on?
The clean-tree operation found and removed the ‘build-1.3/src2’
directory but is not seeing ‘build-1.3/src1’!!

The directory has no special mode or anything.

Regards,
Colin