Dapper.net transaction problem

I'm trying to commit a transaction to my Sql Server 2008 database - firstly 2 insert's followed by a couple update's, however, as soon as it attempts to execute the first of the update's, I get the following error:

ExecuteNonQuery requires the command to have a transaction when the connection assigned to the command is in a pending local transaction. The Transaction property of the command has not been initialized.

Here's the code, edited slightly for brevity:

using (_cn)
{
    _cn.Open();
    IDbTransaction transaction = _cn.BeginTransaction();
    topicId = (int)_cn.Query<decimal>(qAddTopic, new { pForumId = topic.ForumId }, transaction).Single();
    postId = (int)_cn.Query<decimal>(qAddPost, new { pTopicId = topicId }, transaction).Single();

    _cn.Execute(qUpdateForums, new { pLastPostId = postId });
    _cn.Execute((qUpdateSiteTotals));

    transaction.Commit();
}

The first 2 inserts work fine, but as soon as it tries to perform one of the updates, no joy.