title: "no observations from which to fit a model" error
body: <p>I have a data frame with 125 variables.  There are quite a lot of NAs because the data is from a survey which had logic in it to present questions only if it made sense based on previous answers.</p>
  
  <p>I am trying to build a classification tree using the "tree" package.  When I try to build a tree</p>
  
  <pre><code>t1 &lt;- tree(outcome ~ ., data=surveyData)
  </code></pre>
  
  <p>I get the error <code>"no observations from which to fit a model"</code>.</p>
  
  <p>I'm not sure how to interpret this error and haven't found anything on the forums or in the documentation.</p>
  
  <p>I have been trying to build trees with some smaller subsets and that seems to work.</p>
  
  <p>Can someone point me in the right direction?</p>
  
  <p>Many thanks!</p>