"bzr ls FILE" gives exception if branch has no tree

bzr branch https://code.launchpad.net/~mysql/mysql-server/mysql-5.1 mysql-5.1
cd mysql-5.1
bzr ls README
bzr: ERROR: [Errno 20] N'est pas un dossier: '/home/mysql_src/bzrrepos/mysql-5.1/README'
(the French text means it's not a directory).
That is a surprising error; I thought "bzr ls" would imitate "ls", and "ls" works on a file, is not restricted to a directory. By typing such "bzr ls FILE" one could expect seeing the file's existence.
Anyway, even if the error is correct: if I run the same command in a branch created with --no-tree, I get:
Traceback:
 Traceback (most recent call last):
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/commands.py", line 842, in exception_to_return_code
     return the_callable(*args, **kwargs)
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/commands.py", line 1037, in run_bzr
     ret = run(*run_argv)
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/commands.py", line 654, in run_argv_aliases
     return self.run(**all_cmd_args)
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/commands.py", line 1052, in ignore_pipe
     result = func(*args, **kwargs)
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/builtins.py", line 2518, in run
     from_dir=relpath, recursive=recursive):
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/revisiontree.py", line 132, in list_files
     for path, entry in entries:
   File "/home/mysql_src/logiciels/bzr_versions/bzr.dev/bzrlib/inventory.py", line 789, in iter_entries
     children = from_dir.children.items()
 AttributeError: children

Bazaar (bzr) 2.1.0dev3
  from bzr checkout /home/mysql_src/logiciels/bzr_versions/bzr.dev
    revision: 4782
    revid: <email address hidden>