title: Error in if (nc == 0) { : missing value where TRUE/FALSE needed
body: <p>I am running DIF in R and have the following problem after running the code. Please advise me on how I can solve this problem.</p>
  
  <pre><code>library("ordinal")
  
  dat &lt;- read.csv("aaa.csv",header=TRUE)
  dat
  
  
  G &lt;- dat$group
  X &lt;- as.matrix(dat[,2:21])
  
  N &lt;- nrow(X)
  n &lt;- ncol(X)
  
  library("mirt")
  
  SPECS &lt;- mirt.model('F = 1-5
  PRIOR = (1-20, a1, lnorm, 0, 1),
  (1-20,  d1,  norm, 0, 2),
  (1-20,  d2,  norm, 0, 2),
  (1-20,  d3,  norm, 0, 2)  ')
  
  modpoly &lt;- mirt(data=X,model=SPECS,itemtype='graded')
  summary(modpoly)
  coef(modpoly)
  
  T &lt;- fscores(modpoly,method="ML",
               full.scores=TRUE,scores.only=TRUE)
  </code></pre>
  
  <p>IT GIVES THE FOLLOWING ERROR</p>
  
  <blockquote>
    <p>Error in if (nc == 0) { : missing value where TRUE/FALSE needed</p>
  </blockquote>