title: Error in sample.int(length(x), size, replace, prob) : invalid 'size' argument ; when i using evaluationScheme
body: <p>I would like to evaluate my model with the function of the package <code>Recommenderlab</code> </p>
  
  <pre><code>scheme &lt;- evaluationScheme(UserByProductRRM, method = "cross-validation", k = 10, given =-1 , goodRating = 4)
  </code></pre>
  
  <p>but I don't understand why I have this error</p>
  
  <blockquote>
    <p>Error in sample.int(length(x), size, replace, prob) :    invalid
    'size' argument</p>
  </blockquote>