I've written a custom field following the tutorial at http://www.nibble.be/?p=24. The property the field is rendering is an ID for a node (e.g. 1234), and the field renders the name of that node in a span - the field isn't user editable.

Unfortunately, every time I save and publish a document with the custom field, the property backing that field is blanked out - e.g. "1234" becomes "".

public partial class ReadonlyNodeReference : System.Web.UI.UserControl, umbraco.editorControls.userControlGrapper.IUsercontrolDataEditor
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (umbracoValue != null)
            {
                string valStr = umbracoValue.ToString();
                if (!String.IsNullOrWhiteSpace(valStr))
                {
                    int val;
                    if (int.TryParse(valStr, out val))
                    {
                        nodeName.InnerText = getNodeNameFromId(val);
                    }
                    else
                    {
                        nodeName.InnerText = String.Format("Invalid node ID: '{0}'", valStr);
                    }
                }
            }
        }
        private object umbracoValue;
        public object value
        {
            get
            {
                return umbracoValue;
            }
            set
            {
                umbracoValue = value;
            }
        }
        private string getNodeNameFromId(int nodeId)
        {
            if (nodeId > 0)
            {
                var n = new Node((int)nodeId);
                return n.Name;
            }
            else
            {
                return String.Format("Invalid node ID: '{0}'", nodeId);
            }
        }
    }

Does anyone know what I'm doing wrong?