How to pass a 'null' value to INSERT

Hi,

I use a SqlDataAdapter to add a row to a table. I want to set one column
value to be 'null' but it did not show '<null>' on the data grid after I run
the app. It only shows an empty space on the datagrid cell instead of a
'<null>'. The following is my call to set the null value for the 'Date'.
Thanks for help,

CT
DataSet ds = new DataSet("Info");

adapter.Fill(ds, "Info");

DataTable tbl = ds.Tables["Info"];

DataRow row = tbl.NewRow();

row["Id"] = sID;

row["Date"] = null;

tbl.Rows.Add(row);

adapter.Update(ds, "Visitor_Info");