Unable to update column based on variable retrieve

Hello, I have a confirmation function, where when user clicks the link, they are forward to the page that display their confirmation has been approved and the column Status -> varchar(50) is updated with 'YES'. But there seems to be an error, for the column ConfirmID -> uniqueidentifier. I am able to extract the ConfirmID succesfully from the url link, but unable to update STATUS column. I have also set the Status column to null, as to not interfere with other columns that has value already inserted into them. The link is only between the ConfirmID column and the Status column. This is my code:
protected void Page_Load(object sender, EventArgs e) { string userConfirmID = Request.QueryString["ConfirmID"].ToString(); Label2.Text = userConfirmID; string SqlString = "UPDATE test1 SET Status='YES' WHERE ConfirmID=@userConfirmID"; cmd.Parameters.AddWithValue("ConfirmID", userConfirmID); using ...