title: Fill NaN values with letters
body: <p>I have a big dataset, where in one of the columns I want to replace all the NaN values by "ZZZ".
  I have already done this:</p>
  
  <pre><code>df['code_diag_secund_icd10'].fillna("ZZZ", inplace = True)
  </code></pre>
  
  <p>However, when I do</p>
  
  <pre><code>df['code_diag_secund_icd10'].isnull().sum()
  </code></pre>
  
  <p>It gives me the same amount of nan values as before.</p>
  
  <p>Anyone can help me, please?</p>