title: Filtering Pandas Dataframe Aggregate
body: <p>I have a pandas dataframe that I groupby, and then perform an aggregate calculation to get the mean for:</p>
  
  <pre><code>grouped = df.groupby(['year_month', 'company'])
  means = grouped.agg({'size':['mean']})
  </code></pre>
  
  <p>Which gives me a dataframe back, but I can't seem to filter it to the specific company and year_month that I want:</p>
  
  <pre><code>means[(means['year_month']=='201412')]
  </code></pre>
  
  <p>gives me a KeyError</p>