Difference of Lable and String

Hi,
when i use blew code i get Error
cmd.Parameters.Add(New SqlParameter("@url1", Convert.ToString(Request.ServerVariables("HTTP_REFERER")))) But when i change to it:
Label1.Text = Request.ServerVariables("HTTP_REFERER") cmd.Parameters.Add(New SqlParameter("@url1", Label1.Text)) this is work.
How i do? I want use : 
cmd.Parameters.Add(New SqlParameter("@url1", Convert.ToString(Request.ServerVariables("HTTP_REFERER"))))