"unsupported operand type" on "Processing Set patterns" step

[pznamensky]

Processing Set patterns step failed on version 0.1.16 with unsupported operand type error.
RMA installed with pip. Python 3.4.

Processing Set patterns:  43%|████████████████████████████████████▍                                                | 3/7 [00:10<00:14,  3.58s/it]
Traceback (most recent call last):
  File "/home/user/.local/bin/rma", line 9, in <module>
    load_entry_point('rma==0.1.16', 'console_scripts', 'rma')()
  File "/home/user/.local/lib/python3.4/site-packages/rma/cli/rma_cli.py", line 97, in main
    app.run()
  File "/home/user/.local/lib/python3.4/site-packages/rma/application.py", line 143, in run
    str_res.append(self.do_ram(keys))
  File "/home/user/.local/lib/python3.4/site-packages/rma/application.py", line 176, in do_ram
    ret[redis_type] = rule.analyze(keys=aggregate_patterns, total=total_keys)
  File "/home/user/.local/lib/python3.4/site-packages/rma/rule/Set.py", line 84, in analyze
    agg = SetAggregator(progress_iterator((SetStatEntry(x, self.redis) for x in data), progress), len(data))
  File "/home/user/.local/lib/python3.4/site-packages/rma/rule/Set.py", line 43, in __init__
    self.encoding = pref_encoding([obj.encoding for obj in g00], redis_encoding_id_to_str)
  File "/home/user/.local/lib/python3.4/site-packages/rma/rule/Set.py", line 43, in <listcomp>
    self.encoding = pref_encoding([obj.encoding for obj in g00], redis_encoding_id_to_str)
  File "/home/user/.local/lib/python3.4/site-packages/rma/helpers/__init__.py", line 6, in progress_iterator
    for i in x:
  File "/home/user/.local/lib/python3.4/site-packages/rma/rule/Set.py", line 84, in <genexpr>
    agg = SetAggregator(progress_iterator((SetStatEntry(x, self.redis) for x in data), progress), len(data))
  File "/home/user/.local/lib/python3.4/site-packages/rma/rule/Set.py", line 26, in __init__
    self.total = self.valueAlignedBytes + self.system
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

Error occurs only while processing single database. With others one works fine.