title: Pandas KeyError When Merging
body: <p>I'm new to Pandas and i'm trying to do a left join between two Data Frames. I'm getting a KeyError. Could someone spot my mistake? I'd imagine it's probably something easy that i'm missing. Thanks! </p>
  
  <pre><code>import pandas as pd
  import xlrd
  
  
  remediation_file = pd.read_excel('C:\\Users\garyd\\Desktop\\remediation-workbook.xlsx')
  
  exception_file = pd.read_excel('C:\\Users\garyd\\Desktop\\exception-finished-file.xlsx')
  
  
  remediation_file['Concat ID-Resource'] = remediation_file['ID'] + remediation_file['Resource'] 
  
  exception_file['Concat ID-Resource'] = exception_file['ID'] + exception_file['Resource'] 
  
  indexed_remediation_file = remediation_file.set_index(['Concat ID-Resource'])
  
  indexed_exception_file = exception_file.set_index(['Concat ID-Resource'])
  
  # print(indexed_remediation_file)
  
  # print(indexed_exception_file)
  
  
  join_file = pd.merge(indexed_remediation_file, indexed_exception_file, on = 'Concat ID-Resource', how='left')
  
  # print(join_file)
  </code></pre>