title: can't plot rlm-object. non-NA residual length does not match cases used in fitting
body: <p>I'm trying to plot a <code>rlm - object</code> but I'm getting following error</p>
  
  <pre><code>Error in lm.influence(x, do.coef = FALSE) : 
      non-NA residual length does not match cases used in fitting
  </code></pre>
  
  <p>I've searched for this error on the internet, but haven't found anything and I don't understand what it means and why does it appear.</p>
  
  <p>The date which is used in the <code>rlm</code> regression consists out of 1 dependent variable and 4 explanatory variables. none of the observation is <code>NA</code> or <code>NULL</code>.</p>
  
  <p>the result object <code>model$df.residual</code> equals <code>NA</code>. this is the only <code>NA</code> in the whole result object.</p>
  
  <p>Could someone explain me how I could avoid getting the <code>df.residual</code> equal <code>NA</code>?</p>