If the output of the test has a char that isn't ascii, then while saving the output of the test into the stream an UnicodeError is raised on line 277 of core.py (version 0.3.3).
To solve this problem what I did was:
aux = unicode(self._get_err_type(), 'utf-8', 'ignore')
stream.write(aux)
instead of:
stream.write(self._get_err_type())
Hopes this helps....