title: Error message for one model when trying to run two almost identical pgls models
body: <p>I'm trying to create a model using the <code>pgls()</code> function in the package <code>caper</code> to look at the impact of feeding style on shape.  The model below works:</p>
  <pre><code>latdata &lt;- comparative.data(lattree, lat, Species, vcv=TRUE, vcv.dim=3)
  latpigot &lt;- pgls(PC1 + PC2 + PC3 + PC4 + PC5 + PC6 + PC7 ~ Feeding, latdata, lambda = 'ML')
  </code></pre>
  <p>Where <code>lattree</code> is my phylogenetic tree, <code>lat</code> is my dataset and <code>Species</code>, the PCs and <code>Feeding</code> are the names of columns in my dataset.  The PCs are numerical variables with positive and negative values.  <code>Species</code> and <code>Feeding</code> are categorical variables.</p>
  <p>But when I try the same code with a new dataset and new tree it does not work and I get an error:</p>
  <pre><code>vendata &lt;- comparative.data(ventree, ven, Species, vcv=TRUE, vcv.dim=3)
  venpigot &lt;- pgls(PC1 + PC2 + PC3 + PC4 + PC5 + PC6 + PC7 ~ Feeding, vendata, lambda = 'ML')
  
  Error in pgls(PC1 + PC2 + PC3 + PC4 + PC5 + PC6 + PC7 ~ Feeding, vendata,  : 
    Problem with optim:52ERROR: ABNORMAL_TERMINATION_IN_LNSRCH
  </code></pre>
  <p><code>ven</code> has 65 entries instead of the 66 in <code>lat</code>.  The trees are the same except that one of the taxa in <code>lattree</code> is not in <code>ventree</code>.</p>
  <p>From what I understand, the <code>pgls()</code> function has the <code>optim()</code> function somewhere in its code and that's what's causing the issues but I don't understand how the <code>optim()</code> function fits in and why it might be causing issues.</p>
  <p>Does anyone have any ideas as to what might be going wrong?</p>
  <p>Many thanks,</p>
  <p>Carolina</p>
  <p>EDIT:
  If I set <code>lambda</code> to 1, this seems to work.  However, I don't know if this is right or how to decide on appropriate <code>lambda</code> value.  Can anyone help with this?</p>