Set field to null

I am trying to set a field in a sql server table to null if the user hits a certain button.

How can I set it to DBNull?

I have tried the following but It doesnt work.

 Dim updateSOrder As String = "Update SOrders set confirm_id = @dotnet.itags.org.confirmId where SOrder ='" & lblConfirmOrder.Text & "'"
 Dim SaveDelete As SqlCommand
 SaveDelete = New SqlCommand(updateSOrder, oConnection)
 Dim blank As DBNull
 SaveDelete.Parameters.Add(New SqlParameter("@dotnet.itags.org.ConfirmId", SqlDbType.NVarChar, 40))
 SaveDelete.Parameters("@dotnet.itags.org.ConfirmId").Value = blank
 SaveDelete.ExecuteNonQuery()

You may try this:

SaveDelete.Parameters("@.ConfirmId").Value = DBNull.Value