title: fillna() produces NaN values
body: <p>I am using the following code to fill the <code>NaN</code> values and then adding a column to the <code>DataFrame</code> which would contain the number of values in a row which are greater than 0. Here's the code:</p>
  
  <pre><code>df.fillna(0, inplace=True)
  dfMin10 = df
  dfMin10['Sum'] = (dfMin10.iloc[1:len(dfMin10.columns)] &gt; 0).sum(1)
  dfMin10
  </code></pre>
  
  <p>When I see the column <code>Sum</code>, I still see some <code>NaN</code> values. Why would this be? I'm assuming my <code>DataFrame (df)</code> also has some <code>NaN</code> values even after replacing <code>NaN</code>.</p>
  
  <p>Any pointers would be highly appreciated.</p>