cudf cryptic error when printing tail of answer from 1e7 k=2

In 0.8 it was raising segfault. Now in 0.9 it raises following error:

Traceback (most recent call last):
  File "./cudf/groupby-cudf.py", line 56, in <module>
    print(ans.tail(3), flush=True)
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/dataframe.py", line 553, in __str__
    return self.to_string()
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/dataframe.py", line 550, in to_string
    return self.__repr__()
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/dataframe.py", line 591, in __repr__
    output = self.get_renderable_dataframe()
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/dataframe.py", line 582, in get_renderable_dataframe
    output._cols[col].astype("str").str.fillna("null")
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/series.py", line 1383, in astype
    return self._copy_construct(data=self._column.astype(dtype, **kwargs))
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/columnops.py", line 137, in astype
    return self.as_string_column(dtype, **kwargs)
  File "/home/jan/anaconda3/envs/cudf/lib/python3.6/site-packages/cudf/dataframe
/numerical.py", line 129, in as_string_column
    np.dtype(dev_array.dtype)
KeyError: dtype('O')

h2oai / db-benchmark

cudf cryptic error when printing tail of answer from 1e7 k=2 #102