title: pandas df.loc returns blank df
body: <p>I'm sure I'm making an obviously mistake, but can't see it.</p>
  
  <p>I have a df that looks like this:</p>
  
  <pre><code>id year plan grade prior_grade
  21 2017 text A     B
  56 2015 text B     B
  43 2016 text A     C
  </code></pre>
  
  <p>and want to create a new df with only those rows where prior_grade = c.  I'm using this to do so:</p>
  
  <pre><code>prior_c = (df.loc[(df['prior_grade']=='C')])
  </code></pre>
  
  <p>which returns an empty df (column names print but no rows when calling <code>prior_c.head()</code>)</p>
  
  <p>Again, I'm sure I'm making an obvious mistake, but just can't see it.</p>
  
  <p>edit: also tried with less parens and got the same result:</p>
  
  <pre><code>prior_c = df.loc[df['prior_grade']=='C']
  </code></pre>