error while loading page

Jan 5, 2008 at 9:46 AM
when i am my sample facebook application, it shows the following error.

how to fix this.

NullReferenceException: Object reference not set to an instance of an object.
Facebook.Web.FacebookApplication.Render(HtmlTextWriter writer) +140
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +245
System.Web.UI.Page.Render(HtmlTextWriter writer) +37
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean