title: I get this error when I run the following code : '<' not supported between instances of 'NoneType' and 'float'
body: <pre><code>time_periods = [
      'One-Year',
      'Six-month',
      'Three-month',
      'One-month']
  
  for row in hqm_dataframe.index:
  
      for time_period in time_periods:
  
           hqm_dataframe.loc[row,f'{time_period} return percentile'] = stats.percentileofscore(hqm_dataframe[f'{time_period} price return'], hqm_dataframe.loc[row,f'{time_period} price return'])
  </code></pre>
  <p>If someone has a possible solution then it would be really helpful.</p>