title: Nhibernate - Dialect does not support DbType.Double
body: <p>When I try to execute this query:</p>
  
  <pre><code>        var q = session.QueryOver&lt;Member&gt;();
           q.Select(Projections.Avg&lt;Member&gt;(x =&gt; x.AccountBalance));
            var result = q.List();
  </code></pre>
  
  <p>I am getting a:</p>
  
  <pre><code>Dialect does not support DbType.Double
  Parameter name: typecode
  </code></pre>
  
  <p>Any ideas?  I am using a MySQL dialect, and cannot imagine where the query can be wrong as its quite simple.</p>
  
  <p><code>AccountBalance</code> is of type <code>double</code>.  I've even tried it with the average of the <code>ID</code> field, which is of type long, but still got the same exact error message.</p>