bzr merge-upstream crashes if no version is given

If only the upstream tarball is given to merge-upstream, it crashes:

$ bzr mu ../zope.index_3.6.3.orig.tar.gz
Using distribution precise
bzr: ERROR: exceptions.AssertionError:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 923, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1128, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 676, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 698, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/cmds.py", line 784, in run
    assert isinstance(version, str)
AssertionError

This means that no "--version" parameter was given. It should at least print a human-readable error message and exit(1), not assert.

In other case when not enough information is given -- only the version, not the upstream tarball -- bzr-builddeb uses debian/watch to download the needed file. Maybe similar approach (or uscan) can be used here to automatically guess the version number from upstream (or .orig.tar.gz) file name.

ProblemType: Crash
DistroRelease: Ubuntu 12.04
Package: bzr 2.5.0~beta4-1ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-7.13-generic-pae 3.2.0-rc7
Uname: Linux 3.2.0-7-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 1.90-0ubuntu1
Architecture: i386
BzrDebugFlags: set()
BzrVersion: 2.5b4
CommandLine: ['/usr/bin/bzr', 'mu', '../zope.index_3.6.3.orig.tar.gz']
CrashDb: bzr
Date: Tue Jan 3 16:13:31 2012
ExecutablePath: /usr/bin/bzr
FileSystemEncoding: UTF-8
InterpreterPath: /usr/bin/python2.7
Locale: lt_LT.UTF-8
PackageArchitecture: all
Platform: Linux-3.2.0-7-generic-pae-i686-with-Ubuntu-12.04-precise
ProcCmdline: /usr/bin/python /usr/bin/bzr mu ../zope.index_3.6.3.orig.tar.gz
PythonVersion: 2.7.2
SourcePackage: bzr
Title: bzr crashed with AssertionError in run()
UpgradeStatus: Upgraded to precise on 2011-12-29 (4 days ago)
UserEncoding: UTF-8
UserGroups: adm admin cdrom dialout dip libvirtd lpadmin plugdev sambashare