title: Pandas merge TypeError: object of type 'NoneType' has no len()
body: <p>I'm experimenting with pandas merge left_on and right_on params.
  According to <a href="https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html" rel="noreferrer">Documentation 1</a> and <a href="https://pandas.pydata.org/pandas-docs/stable/merging.html" rel="noreferrer">Documentation 2</a></p>
  
  <p>Documentation 1: states that left_on and right_on are field names to join on in left/right DataFrame.
  Documentation 2: Columns from the left DataFrame to use as <strong>keys</strong></p>
  
  <p>What does <strong>keys</strong> means?</p>
  
  <p>Following documentation 1:</p>
  
  <pre><code>left_frame = pd.DataFrame({'key': range(5),
                         'left_value': ['a', 'b', 'c', 'd', 'e']})
  right_frame = pd.DataFrame({'key': range(2,7),
                         'right_value': ['f', 'g', 'h', 'i', 'j']})
  </code></pre>
  
  <p>I did this:</p>
  
  <pre><code>df = pd.merge(left_frame,right_frame,how='right',right_on='key')
  </code></pre>
  
  <p>left_frame has 'key' as field name, but yet it returns </p>
  
  <pre><code>TypeError: object of type 'NoneType' has no len()
  </code></pre>