title: stl function in R is not recognising a univariate time series
body: <p>I am using R to read a time series from Excel (using XLConnect), then running some forecast models on that time series, and then outputting the results back to Excel.  It's a long story, but the company I'm doing a Masters by Research for want to keep using Excel!  Anyhow, I can extract the time series I want from Excel.  I use ts() to make it a time series.  I then run forecasts on that series using (in this order) <code>ets()</code>, <code>auto.arima()</code>, <code>tbats()</code>, <code>mapaest()</code>, <code>theta()</code>, and finally <code>stlf()</code>.  To check that it's doing the forecasts I get R to print off the results of the forecasts.  It runs through all the forecast models fine until it gets to the <code>stlf()</code> forecast function.  When I get this error:</p>
  
  <blockquote>
    <p>Error in stl(x, s.window = s.window, t.window = t.window, robust =
    robust) :    only univariate series are allowed</p>
  </blockquote>
  
  <p>My question is how come the time series (it is univariate) works fine in the other forecast functions but not in the <code>stlf()</code> function?</p>