title: spatstat: plot.msr error message
body: <p>I have created an error free ppm in <code>spatstat</code> (R library) together with the associated residuals.  It is based around approximately 25,000 data points.  However, when I attempt to plot the residuals using plot.msr I get the following message:</p>
  
  <pre><code>Error: all(sigma &gt; 0) is not TRUE
  </code></pre>
  
  <p>Any suggestions as to what this means and for what I can do to sort out the problem</p>
  
  <p>Thanks</p>