qconfig : "Set default" in the merge tab raises exception.

In Merge tab in qconfig window, There is "Set default" button.

1. Clicking it raises following exception.
bzr: ERROR: exceptions.AttributeError: 'str' object has no attribute 'name'

Traceback (most recent call last):
  File "D:\DEVELOP\BZR_PLUGINS\qbzr\lib\config.py", line 669, in merge_tools_set_default_clicked
    self.merge_tools_model.set_default(self.get_selected_merge_tool().name)
AttributeError: 'str' object has no attribute 'name'

2. In the first place, why this button exist ?
    Default tool seems to be specified by checkbox in the list.