Explorer Won't Launch

I have just installed Bazaar Explorer on my Windows 7 Pro computer, but when I attempt to launch it nothing appears to happen. The output to my log file is:

0.041 bazaar version: 2.5.0
0.041 bzr arguments: [u'explorer']
0.045 looking for plugins in C:/Users/<Username Removed>/AppData/Roaming/bazaar/2.0/plugins
0.046 looking for plugins in C:/Program Files (x86)/Bazaar/plugins
0.046 Plugin name explorer already loaded
0.102 encoding stdout as osutils.get_user_encoding() 'cp1252'
0.330 Traceback (most recent call last):
  File "bzrlib\commands.pyo", line 920, in exception_to_return_code
  File "bzrlib\commands.pyo", line 1131, in run_bzr
  File "bzrlib\commands.pyo", line 673, in run_argv_aliases
  File "bzrlib\commands.pyo", line 695, in run
  File "bzrlib\cleanup.pyo", line 136, in run_simple
  File "bzrlib\cleanup.pyo", line 166, in _do_with_cleanups
  File "C:/Users/<Username Removed>/AppData/Roaming/bazaar/2.0/plugins\explorer\lib\commands.py", line 161, in run
  File "bzrlib\lazy_import.pyo", line 108, in __getattribute__
  File "bzrlib\lazy_import.pyo", line 85, in _resolve
  File "bzrlib\lazy_import.pyo", line 200, in _import
  File "C:/Users/<Username Removed>/AppData/Roaming/bazaar/2.0/plugins\explorer\lib\explorer.py", line 29, in <module>
ImportError: cannot import name shlex_split_unicode

0.338 couldn't find apport bug-reporting library: No module named apport
0.346 return code 4

What would cause a problem like this?