title: Warning message: "missing values in resampled performance measures" in caret train() using rpart
body: <p>I am using the caret package to train a model with "rpart" package;</p>
  
  <pre><code>tr = train(y ~ ., data = trainingDATA, method = "rpart")
  </code></pre>
  
  <p>Data has no missing values or NA's, but when running the command a warning message comes up; </p>
  
  <pre><code>    Warning message:
  In nominalTrainWorkflow(x = x, y = y, wts = weights, info = trainInfo,  :
    There were missing values in resampled performance measures.
  </code></pre>
  
  <p>Does anyone know (or could point me to where to find an answer) what does this warning mean?
  I know it is telling me that there were missing values in resampled performance measures - but what does that exactly mean and how can a situation like that arise?
  BTW, the <code>predict()</code>  function works fine with the fitted model, so it is just my curiosity.</p>