title: How to solve the Pandas issue related to Series.fillna()?
body: <p>I just upgrade from Pandas 0.11 to 0.13.0rc1. The upgration caused one error related to Series.fillna().</p>
  
  <pre><code>&gt;&gt;&gt; df
                     sales  net_pft
  STK_ID RPT_Date                  
  600809 20060331   5.8951   1.1241
         20060630   8.3031   1.5464
         20060930  11.9084   2.2990
         20061231      NaN   2.6060
         20070331   5.9129   1.3334
  
  [5 rows x 2 columns]
  &gt;&gt;&gt; type(df['sales'])
  &lt;class 'pandas.core.series.Series'&gt;
  &gt;&gt;&gt; df['sales'] = df['sales'].fillna(df['net_pft'])
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "D:\Python27\lib\site-packages\pandas\core\generic.py", line 1912, in fillna
      obj.fillna(v, inplace=True)
  AttributeError: 'numpy.float64' object has no attribute 'fillna'
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Why <code>df['sales']</code> become <code>'numpy.float64'</code> object when it is used in <code>fillna()</code> ? How to correctly do "fill the NaN of one column with the other column's value" ?</p>