title: Grouping variable must be a vector or a character array
body: <p>I have done the following:</p>
  
  <pre><code>I=imread('image.png');
  I2=im2double(I);
  cidx = fcm(I2,5);
  </code></pre>
  
  <p>When I tried running the following command:</p>
  
  <pre><code> silhouette(I2,cidx)
  </code></pre>
  
  <p>I got the following error:</p>
  
  <pre><code>Error using grp2idx (line 39)
  Grouping variable must be a vector or a character array.
  
  Error in silhouette (line 79)
  [idx,cnames] = grp2idx(clust);
  </code></pre>
  
  <p>How can I solve this issue?</p>
  
  <p>Thanks.</p>