title: Column missing after Pandas GroupBy (not the GroupBy column)
body: <p>I am using the following source code:</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  
  
  # Load data
  data = pd.read_csv('C:/Users/user/Desktop/Daily_to_weekly.csv', keep_default_na=True)
  
  print(data.shape[1])
  # 18
  
  # Create weekly data
  # Agreggate by calculating the sum per store for every week
  data_weekly = data.groupby(['STORE_ID', 'WEEK_NUMBER'], as_index=False).agg('sum')
  
  print(data_weekly.shape[1])
  # 17 
  </code></pre>
  
  <p>As you may see for some reason a column is missing after the aggregation and this column is neither of the GroupBy columns (<code>'STORE_ID', 'WEEK_NUMBER'</code>).</p>
  
  <p>Why is this happening and how can I fix it?</p>