title: Error in na.fail.default: missing values in object - but no missing values
body: <p>I am trying to run a lme model with these data:</p>
  
  <pre><code>tot_nochc=runif(10,1,15)
  cor_partner=factor(c(1,1,0,1,0,0,0,0,1,0))
  age=runif(10,18,75)
  agecu=age^3
  day=factor(c(1,2,2,3,3,NA,NA,4,4,4))
  dt=as.data.frame(cbind(tot_nochc,cor_partner,agecu,day))
  attach(dt)
  
  corpart.lme.1=lme(tot_nochc~cor_partner+agecu+cor_partner *agecu, 
                    random = ~cor_partner+agecu+cor_partner *agecu |day, 
                    na.exclude(day))
  </code></pre>
  
  <p>I get this error code:</p>
  
  <blockquote>
    <p>Error in na.fail.default(list(cor_partner = c(1L, 1L, 2L, 1L, 1L, 1L,  : 
      missing values in object</p>
  </blockquote>
  
  <p>I am aware there are similar questions in the forum. However, in my case:  </p>
  
  <ul>
  <li>cor_partner has no missing values;  </li>
  <li>the whole object is coded as a factor (at least from what the Global Environment shows).</li>
  </ul>
  
  <p>I could exclude those NA values with an na.action, but I'd rather know why the function is reading missing values - to understand exactly what is happening to my data.</p>