title: Pandas series .to_sparse() raises: TypeError("cannot concatenate a non-NDFrame object")
body: <p>I have a Pandas Series object (<a href="https://github.com/UdyatGithub/PyntCloud/blob/master/pyntcloud/SO_issue.csv" rel="nofollow">LINK TO DATA EXAMPLE</a>):</p>
  
  <pre><code>type(ts)
  Out[54]: pandas.core.series.Series
  
  ts.isnull().sum()
  Out[16]: 2744
  
  ts.size
  Out[17]: 4096
  
  ts.dtype
  Out[19]: dtype('float64')
  </code></pre>
  
  <p>And I'm getting this error when I try to convert the series to a <a href="http://pandas.pydata.org/pandas-docs/stable/sparse.html" rel="nofollow">Sparse data structure</a> by using <code>to_sparse</code></p>
  
  <pre><code>ts.to_sparse()
  Traceback (most recent call last):
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/IPython/core/formatters.py", line 699, in __call__
      printer.pretty(obj)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/IPython/lib/pretty.py", line 383, in pretty
      return _default_pprint(obj, self, cycle)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/IPython/lib/pretty.py", line 503, in _default_pprint
      _repr_pprint(obj, p, cycle)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/IPython/lib/pretty.py", line 694, in _repr_pprint
      output = repr(obj)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/base.py", line 63, in __repr__
      return str(self)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/base.py", line 42, in __str__
      return self.__unicode__()
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/sparse/series.py", line 287, in __unicode__
      series_rep = Series.__unicode__(self)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/series.py", line 959, in __unicode__
      max_rows=max_rows)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/series.py", line 1000, in to_string
      dtype=dtype, name=name, max_rows=max_rows)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/series.py", line 1027, in _get_repr
      max_rows=max_rows)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/format.py", line 144, in __init__
      self._chk_truncate()
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/core/format.py", line 158, in _chk_truncate
      series.iloc[-row_num:]))
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/tools/merge.py", line 834, in concat
      copy=copy)
  
    File "/home/david/anaconda3/lib/python3.5/site-packages/pandas/tools/merge.py", line 890, in __init__
      raise TypeError("cannot concatenate a non-NDFrame object")
  
  TypeError: cannot concatenate a non-NDFrame object
  </code></pre>
  
  <p>I really don't understand why this error is raising and I would like to know what is happening.</p>