Cannot convert anonymous method to type 'System.Delegate' because it is not a delegate type

I want to execute this code on the main thread in the WPF app and getting an error I can't figure out what is wrong:

private void AddLog(string logItem)
{
    this.Dispatcher.BeginInvoke(
        delegate()
            {
                this.Log.Add(new KeyValuePair<string, string>(DateTime.Now.ToLongTimeString(), logItem));
            });
}