title: Nans not removed after dropna
body: <p>i have a dataframe of 400 x 25. i am supposed to remove ALL Nans from the dataset and then select only 3 columns to work on from the 25 columns. I have done this using:</p>
  
  <pre><code>df1.dropna(axis=0)
  df2=df1.loc[:,['bgr','wc','rc']]
  df2['rc']=pd.to_numeric(df['rc'],errors='coerce')
  df2['wc']=pd.to_numeric(df['wc'],errors='coerce')
  </code></pre>
  
  <p>rc and wc were showing up as objects when i did the dtypes, so i converted them to floats.</p>
  
  <p>now, after i drop the Nans, i am left with only 252 rows. so i assumed that all the Nans had gone.</p>
  
  <p>but when i did a df2.describe(), it showed 251 as count for 'rc' but 252 for 'bgr' and 'wc'. this was odd, so when i did a df2.rc.unique(), it showed up a Nan!!!!</p>
  
  <p>my question: how do i remove this Nan??</p>