title: 'numpy.ndarray' object is not callable error
body: <p>Hi I am getting the following error</p>
  
  <p>'numpy.ndarray' object is not callable</p>
  
  <p>when performing the calculation in the following manner</p>
  
  <pre><code>rolling_means = pd.rolling_mean(prices,20,min_periods=20)`
  rolling_std =  pd.rolling_std(prices, 20)`
  
  #print rolling_means.head(20)
   upper_band = rolling_means + (rolling_std)* 2
   lower_band = rolling_means - (rolling_std)* 2
  </code></pre>
  
  <p>I am not sure how to resolve, can someone point me in right direction....</p>