title: pandas groupby() receiving error message "level > 0 only valid with MultiIndex"
body: <p>I'm trying to perform a simple groupby operation but it's receiving an error message "level > 0 only valid with MultiIndex" and being a python newbie, I don't understand what that means or know where the error is. </p>
  
  <p>In my data frame di, I have three variables disabled, wpfinwgt, and rhcalmn, whose values run from 1 to 12 (one for each calender month). I want to sum the two variables by rhcalmn. the code I have is:</p>
  
  <pre><code>di_bymonth=di.groupby(level=['rhcalmn']).sum()[['disabled','wpfinwgt']]
  </code></pre>
  
  <p>and I get </p>
  
  <pre><code>ValueError: level &gt; 0 only valid with MultiIndex
  </code></pre>
  
  <p>What is wrong with it? Thanks for your help.</p>