'utf8' codec can't decode byte 0x94 in position 17: invalid start byte

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/ajenti/plugins/main/main.py", line 163, in handle_message
    self.send_ui()
  File "/usr/lib/pymodules/python2.7/ajenti/plugins/main/main.py", line 192, in send_ui
    data = json.dumps(self.ui.render())
  File "/usr/lib/python2.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 263, in iterencode
    return _iterencode(o, 0)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x94 in position 17: invalid start byte