title: 'Float' object has no attribute 'log'
body: <p>I have a time series with price information in column <code>price</code>.  When I tried to create a new column <code>ln_price</code> by taking the <code>ln</code> of column <code>price</code> I got an error: </p>
  
  <blockquote>
    <p>AttributeError: 'float' object has no attribute 'log'</p>
  </blockquote>
  
  <p>Can someone help me understand why this would be and how it can be fixed?</p>
  
  <p>Thanks!</p>
  
  <pre><code>df['ln_price'] = np.log(df['price'])
  </code></pre>