title: Strangely empty DataFrame after groupby and agg
body: <p>With pandas I want to do something very similar to</p>
  
  <pre><code>import pandas as pd
  
  dict1 = {((('Message1',),('Message1','Message4',)),('Message4',)): 10}
  dict2 = {(1677, 468): 3}
  dict3 = {(1677, 468): 1, (2078, 869): 1 }
  
  df1 = pd.DataFrame.from_dict(dict1, orient='index'). \
      reset_index(). \
      rename(columns={'index': 'pair', 0: 'pair_count'})
  
  df2 = pd.DataFrame.from_dict(dict2, orient='index'). \
      reset_index(). \
      rename(columns={'index': 'pair', 0: 'pair_count'})
  
  df3 = pd.DataFrame.from_dict(dict3, orient='index'). \
      reset_index(). \
      rename(columns={'index': 'pair', 0: 'pair_count'})
  
  pd.concat([df1, df2, df3]).\
      groupby('pair').\
      sum()
  </code></pre>
  
  <p>If you run the code above, this should work. However with my data, which looks very similar, but longer in regards of dict2 and dict3, this does not work. I am able to concatenate the DataFrames, but then when I do the grouping and the sum, I receive an empty DataFrame. </p>
  
  <p>Unfortunately, when I try to reproduce the error by extracting the data, this does work. (This is why I am not able to post an example where the error occurs.) So I guess this issue is not about the data itself... maybe, because here is the weirdest thing: If I change dict1 to</p>
  
  <pre><code>dict1 = {((('Message1',),('Message1',)),('Message4',)): 10}
  </code></pre>
  
  <p>I get the desired result.</p>