title: error in stl .series is not periodic
body: <p>I am pretty sure I am missing something which is very simple but still not able to figure out why this error is showing up .
  The data I have is of every month end data from 2013 Apr to 2014 Mar. Now I want to understand the trend over the 12 months period . </p>
  
  <pre><code>xx &lt;- structure(c(41.52, 41.52, 41.52, 41.68, 41.68, 41.68, 41.84, 
  41.84, 41.84, 42.05, 42.05, 42.05), .Tsp = c(2013.25, 2014.16666666667, 
  12), class = "ts");
  </code></pre>
  
  <p>is my time series data .
  Now when I use </p>
  
  <pre><code> stl(xx,s.window ="periodic")
  </code></pre>
  
  <p>I get error :</p>
  
  <pre><code>Error in stl(xx, s.window = "periodic") : 
    series is not periodic or has less than two periods
  </code></pre>
  
  <p>i am not sure what is wrong as I understand the series has 12 periods .Please assist </p>