title: df.fillna(0) command won't replace NaN values with 0
body: <p>I'm trying to replace the NaN values generated in the code below to 0. I don't understand what the below won't work. It still keeps the NaN values.</p>
  
  <pre><code>df_pubs=pd.read_sql("select Conference, Year, count(*) as totalPubs from publications where year&gt;=1991 group by conference, year", db)
  
  df_pubs['Conference'] = df_pubs['Conference'].str.encode('utf-8')
  
  df_pubs = df_pubs.pivot(index='Conference', columns='Year', values='totalPubs')
  df_pubs.fillna(0)
  
  print df_pubs
  </code></pre>
  
  <p><code>print df produces</code> this:</p>
  
  <pre><code>Year                                                                                       1991  \
  Conference                                                                                        
                                                                                              223   
  10th Anniversary Colloquium of UNU/IIST                                                     NaN   
  15. WLP                                                                                     NaN   
  1999 ACM SIGMOD Workshop on Research Issues in Data Mining and Knowledge Discovery          NaN   
  25 Years CSP                                                                                NaN  
  </code></pre>