Custom Error Page not opening as Model Dialog Box

Hi,

I have created a custom Error Page to show the message on Item Deleting Event Receiver.

public override void ItemDeleting(SPItemEventProperties properties) 
{
    base.ItemDeleting(properties); 
        properties.Cancel = true; 
        properties.Status = SPEventReceiverStatus.CancelWithRedirectUrl; 
        properties.RedirectUrl = "/_layouts/EventReceiverProject1/CustomErrorPage.aspx
    }
}

when I click on delete option from ribbon it redirects me to the customErrorPage. but it does now show customErrorPage as Model window. it shows me the full page with top and left navigation. how can I open customErrorPage in model window.

please help.