title: Why does Mahout RMSRecommenderEvaluator evaluate method sometimes results NaN?
body: <p>Im trying to evaluate my recommendation system using the following code </p>
  
  <p>RecommenderEvaluator rmsEvaluator = new RMSRecommenderEvaluator();
  double score = rmsEvaluator.evaluate(recommenderBuilder, null, model, 0.95, 0.05);
  System.out.println("RMS Evaluator Score: " + score);</p>
  
  <p>Some time the score is NaN.</p>
  
  <p>Why does Mahout RMSRecommenderEvaluator evaluate method results NaN?</p>