`bst source-bundle` doesn't work

Running bst source-bundle my.bst results in:

Traceback (most recent call last):
  File "/usr/local/bin/bst", line 11, in <module>
    load_entry_point('BuildStream==1.0', 'console_scripts', 'bst')()
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/buildstream/_frontend/main.py", line 416, in source_bundle
    compression, except_, directory)
  File "/usr/local/lib/python3.5/dist-packages/buildstream/_pipeline.py", line 763, in source_bundle
    element._write_script(source_directory)
  File "/usr/local/lib/python3.5/dist-packages/buildstream/element.py", line 627, in _write_script
    commands=self.generate_script()
  File "/usr/local/lib/python3.5/dist-packages/buildstream/buildelement.py", line 217, in generate_script
    commands = self.commands[command_name]
AttributeError: 'ImportElement' object has no attribute 'commands'