title: Dataframe mean returns empty series
body: <p>I have a dataframe as follows. I want to get the mean of column <code>pred1_score</code> as a number. However, my attempt returns an empty series. Could you please help me?</p>
  <pre><code>&gt;&gt;&gt; df1.head()
     Unnamed: 0 Unnamed: 0.1 compare  ... preds_score pred1_score pred2_score
  0         0.0            0      //  ...      0.6045      0.6195      0.5272
  1         1.0            1      //  ...      0.6045      0.5403      0.4759
  2         2.0            2      //  ...      0.6045      0.3759      0.3636
  3         3.0            3      //  ...      0.6045      0.5923      0.5187
  4         4.0            4      //  ...      0.6045      0.5926      0.4553
  
  [5 rows x 13 columns]
  &gt;&gt;&gt; df1[[&quot;pred1_score&quot;]].mean()
  Series([], dtype: float64)
  </code></pre>
  <p>I also tried:</p>
  <pre><code>df1[['pred1_score']].astype(float).mean()
  </code></pre>
  <p>But it throws:</p>
  <pre><code>ValueError: could not convert string to float: 'pred1_score'
  </code></pre>
  <p>Which could be the column head.</p>