title: seasonal_decompose: operands could not be broadcast together with shapes on a series
body: <p>I know there are many questions on this topic, but none of them helped me to solve this problem. I'm really stuck on this.</p>
  
  <p>With a simple series:</p>
  
  <pre><code>0
  2016-01-31  266
  2016-02-29  235
  2016-03-31  347
  2016-04-30  514
  2016-05-31  374
  2016-06-30  250
  2016-07-31  441
  2016-08-31  422
  2016-09-30  323
  2016-10-31  168
  2016-11-30  496
  2016-12-31  303
  
  import statsmodels.api as sm
  logdf = np.log(df[0])
  decompose = sm.tsa.seasonal_decompose(logdf,freq=12, model='additive')
  decomplot = decompose.plot()
  </code></pre>
  
  <p>i keep getting: <code>ValueError: operands could not be broadcast together with shapes (12,) (14,)</code> </p>
  
  <p>I've tried pretty much everything, passing only logdf.values, passing a non-log series. It doesn't work.
  Numpy and statsmodel versions:</p>
  
  <pre><code>print(statsmodels.__version__)
  print(pd.__version__)
  print(np.__version__)
  0.6.1
  0.18.1
  1.11.3
  </code></pre>