System.NullReferenceException

Hello Community -- I have a .NET 2003 app that I converted to .NET 2005. I'm receiving an error message "System.NullReferenceException" when I complie. Here's the code -- Thank you in advance.

private DataTable GetWeekDays(){
 string query = "SELECT * from dbo.fnGetWeekDay('" + selection.ToShortDateString() + "') order by Seq";

DataSet ds = SqlHelper.ExecuteDataset(Application["DBConn"].ToString(), CommandType.Text, query);
return ds.Tables[0];
            }//GetWeekDays()