title: Key error when trying to vlookup pandas
body: <p>I am trying to vlookup values of EW against AD.  </p>
  
  <p>Dataframe 1</p>
  
  <pre><code>    EW      
  0   A  
  1   BC   
  </code></pre>
  
  <p>Dataframe 2</p>
  
  <pre><code>      AD     
  0     A  
  1     B  
  2     BC  
  </code></pre>
  
  <p>Then I run:</p>
  
  <pre><code>df3 = df1.insert(0, 'AD', df1['EW'].map(df2.set_index('EW')['AD']))
  print (df1)
  </code></pre>
  
  <p>Expect output for df3:</p>
  
  <pre><code>      AD     
  0     A  
  1     NaNa  
  2     BC  
  </code></pre>
  
  <p>I keep getting: <code>KeyError: 'EW'</code></p>