title: warning message '+' not meaningful for factors
body: <p>Using for self-defined variables, I am receiving the following error when trying to run odds ratio analysis.</p>
  
  <pre><code>dat1 &lt;- escalc(measure="OR", ai=a, bi=b, ci=c, di=d, data=dat.turnover)
  warnings(dat1)
  </code></pre>
  
  <blockquote>
    <p>Warning messages:
    1: In Ops.factor(ai, bi) : ‘+’ not meaningful for factors 
    Error in cat(list(...), file, sep, fill, labels, append) : 
      argument 2 (type 'list') cannot be handled by 'cat'</p>
  </blockquote>
  
  <p>All the data I am using for <code>a</code>, <code>b</code>, <code>c</code>, and <code>d</code> are numeric.  </p>