'bzr-buildpackage --merge' fails needlessly when there are no 'source' URIs in sources.list

Having just placed an upstream tarball in my tarballs directory, I was somewhat surprised to receive the following error. Better behaviour would be to try subsequent tarball sources.

$ bzr-buildpackage --merge
Building using working tree
Running in merge mode
Looking for a way to retrieve the upstream tarball
bzr: ERROR: exceptions.SystemError: E:You must put some 'source' URIs in your sources.list

Traceback (most recent call last):
  File "/home/dwa/devel/bzr.dev/bzrlib/commands.py", line 727, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/dwa/devel/bzr.dev/bzrlib/commands.py", line 922, in run_bzr
    ret = run(*run_argv)
  File "/home/dwa/devel/bzr.dev/bzrlib/commands.py", line 559, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/dwa/.bazaar/plugins/builddeb/cmds.py", line 396, in run
    builder.export()
  File "/home/dwa/.bazaar/plugins/builddeb/builder.py", line 74, in export
    self.distiller.distill(self.target_dir)
  File "/home/dwa/.bazaar/plugins/builddeb/source_distiller.py", line 82, in distill
    self._distill(target)
  File "/home/dwa/.bazaar/plugins/builddeb/source_distiller.py", line 123, in _distill
    tarball = self.upstream_provider.provide(parent_dir)
  File "/home/dwa/.bazaar/plugins/builddeb/upstream.py", line 220, in provide
    elif self.provide_with_apt(self.store_dir):
  File "/home/dwa/.bazaar/plugins/builddeb/upstream.py", line 253, in provide_with_apt
    target_dir)
  File "/home/dwa/.bazaar/plugins/builddeb/upstream.py", line 59, in provide_with_apt
    sources = apt_pkg.GetPkgSrcRecords()
SystemError: E:You must put some 'source' URIs in your sources.list

bzr 1.14dev on python 2.5.4 (linux2)
arguments: ['/home/dwa/bin/bzr', 'builddeb', '--merge']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_GB.UTF-8'
plugins:
  builddeb /home/dwa/.bazaar/plugins/builddeb [unknown]
  bzrtools /home/dwa/.bazaar/plugins/bzrtools [1.14]
  fastimport /home/dwa/.bazaar/plugins/fastimport [unknown]
  gtk /home/dwa/.bazaar/plugins/gtk [0.96.0.dev.1]
  launchpad /home/dwa/devel/bzr.dev/bzrlib/plugins/launchpad [unknown]
  loom /home/dwa/.bazaar/plugins/loom [1.4dev]
  netrc_credential_store /home/dwa/devel/bzr.dev/bzrlib/plugins/netrc_credential_store [unknown]
  rebase /home/dwa/.bazaar/plugins/rebase [0.4.1]
  stats /home/dwa/.bazaar/plugins/stats [unknown]
  svn /home/dwa/.bazaar/plugins/svn [0.5.4dev]
  upload /usr/lib/python2.5/site-packages/bzrlib/plugins/upload [0.1]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.