title: Key Error Pandas pd.isnull
body: <p>Part of my code looks like this:</p>
  
  <pre><code>#generate exceptions CSV where ascomb['Discipline']=='""'
  
  #ascombnb: THESE ARE THE MERGED, NON-BLANK ASSESSMENTS AND STUDENTS RECORDS
  ascombnoa = ascomb[pd.isnull(ascomb['Discipline'])]
  ascombnoa.to_csv(str(out_folder)+'\Exceptions\Students_Without_Assessments'+str(st)+'.csv', sep=',')
  ascombnos = ascomb[pd.isnull(ascomb['Grade'])]
  ascombnos.to_csv(str(out_folder)+'\Exceptions\Assessments_Without_Students'+str(st)+'.csv', sep=',')
  </code></pre>
  
  <p>I have been using this code for months and it never produced this key error (it's associated with 'Discipline'. </p>