title: Pandas dataframe raises KeyError when sort_values() method is called
body: <p>I'm new at pandas and trying to simply learn something about it and it's dataframes. I want to sort data using one column, but when I tried to use:</p>
  
  <pre><code> print(df.sort_values(by = 'avg_low'))
  </code></pre>
  
  <p>and</p>
  
  <pre><code>print(df.sort_values('avg_low'))
  </code></pre>
  
  <p>but it always throws KeyError. ('avg_low' is column's name)
  My data is: </p>
  
  <pre><code>month  avg_high  avg_low  record_high  record_low  avg_precipitation
  Jan        58       42           74          22               2.97
  Feb        34       42           74          22               2.97
  Mar        54       42           74          22               1.97
  Apr        65       42           74          21               2.97
  May        32       42           74          22               3.32
  </code></pre>
  
  <p>what can I do to sort this somehow?</p>