title: pandas Python Series objects are mutable, thus they cannot be hashed in query method
body: <p>I'm tring to run:</p>
  
  <pre><code>import pandas as pd
  df_data = pd.DataFrame({'a':[1,20,None,40,50]})
  df_data.query('a.isnull()')
  </code></pre>
  
  <p>and an error happens :</p>
  
  <pre><code>TypeError: 'Series' objects are mutable, thus they cannot be hashed
  </code></pre>
  
  <p>but:</p>
  
  <pre><code>df_data.a.isnull()
  </code></pre>
  
  <p>has no error at all,
  why is this happening?
  can you help me to figure out why?</p>