title: In Pandas, How to use fillna to fill the whole columns with string if the column is empty originally?
body: <p>My table:</p>
  
  <pre><code>In [15]: csv=u"""a,a,,a
     ....: b,b,,b
     ....: c,c,,c
     ....: """
  
  In [18]: df = pd.read_csv(io.StringIO(csv), header=None)
  </code></pre>
  
  <p>Fill the empty columns as 'UNKNOWN'</p>
  
  <pre><code>In [19]: df
  Out[19]: 
     0  1   2  3
  0  a  a NaN  a
  1  b  b NaN  b
  2  c  c NaN  c
  
  In [20]: df.fillna({2:'UNKNOWN'})
  </code></pre>
  
  <p>Got the error</p>
  
  <pre><code>ValueError: could not convert string to float: UNKNOWN
  </code></pre>