title: #N/A values are getting treated as zero in python pandas dataframe
body: <p>I have an excel sheet which has values like <code>#N/A</code>. I want them to be treated as Null(np.nan) while loading the excel into <code>dataframe</code> but pandas <code>dataframe</code> is converting them to 0 value. </p>
  
  <p>Kindly suggest what's wrong with the below code:</p>
  
  <pre><code>df=pd.read_excel('sample.xls',header=2,keep_default_na=False,na_values=[''])
  </code></pre>
  
  <p>Thanks !</p>