title: Traceback KeyError when Entrez increases retmax
body: <p>I am attempting to gather a list of pubmed articles using biopython entrez. I want to gather certain parts of the article from the medline format. The code I have wrote below works if there is no retmax set. It defaults to 20 articles, however, I want to gather a much larger amount of articles. If I set the retmax to a higher number I receive the error below.</p>
  
  <pre><code>#!/usr/bin/env python
  from Bio import Entrez, Medline
  
  Entrez.email = "foobar@gmail.edu"    
  handle = Entrez.esearch(db="pubmed",
                          term="stanford[Affiliation]", retmax=1000)
  record = Entrez.read(handle)
  pmid_list = record["IdList"]
  
  more_data = Entrez.efetch(db="pubmed", id=",".join(pmid_list), rettype="medline", retmode="text")
  all_records = Medline.parse(more_data)
  
  record_list = []
  for record in all_records:
      record_dict = {'ID': record['PMID'],
                      'Title': record['TI'],
                      'Publication Date': record['DP'],
                      'Author': record['AU'],
                      'Institute': record['AD']}
      record_list.append(record_dict)
  </code></pre>
  
  <p>I then receive the error</p>
  
  <pre><code>Traceback (most recent call last):
    File "./pubmed_pull.py", line 42, in &lt;module&gt; 
      'Institute': record['AD']}
  KeyError: 'AD'
  </code></pre>
  
  <p>I am unsure why I get an error if I increase the number of articles.</p>