title: Error "No class variable" in mice.2l.norm
body: <p>I hope you are well. I am having some trouble with mice in R with a code that had previously ran without a problem and is now not running. I am trying to use 2-level imputation and I am defining my predictor matrix as: </p>
  
  <pre><code>pred["sbp_adult",]&lt;-c(0,0,0,0,2,
                        2,2,2,2,2,
                        2,2,2,2,2,
                        2,2,2,0,0,
                        0,0,0,0,0,
                        0,0,0,0,0,
                        -2,0,0)
  
  
  
  imp2 &lt;- mice(data_imp, meth=c('','', '','2l.norm','2l.norm',
                                 '','','','2l.norm','2l.norm',
                                 '2l.norm','2l.norm','','','2l.norm',
                                 '2l.norm','','','2l.norm','2l.norm',
                            '2l.norm','2l.norm','2l.norm','2l.norm','2l.norm',
                                 '2l.norm','2l.norm','','','', '', '', ''), 
                pred=pred, maxit=1, m=numimp, seed=seeds)
  </code></pre>
  
  <p>The error message that I get is : </p>
  
  <blockquote>
    <p>1   1  sbp_adult Error in mice.impute.2l.norm(c(127, 103.25, 120.5, 103, 108.25, 112, 116,  : 
      No class variable</p>
  </blockquote>
  
  <p>But there is clearly a class variable in the pred matrix as -2. What am I doing wrong? 
  Thank you! 
  m</p>