Web Forms :: Can't Add Item To Different Dropdownlist By Passing Different Parameter?

call the method by passing parameter:

add_ddl_item_by_dt((DataTable)Session["dt_1"],ddl_1);
add_ddl_item_by_dt((DataTable)Session["dt_2"],ddl_2);
protected void add_ddl_item_by_dt(DataTable dt,DropDownList ddl)
{
for (int i = 0; i < dt.Rows.Count; i++)
{
ddl.Items.Add(dt.Rows[i]["Field_1"].ToString());
}
}

why i cannot use the above method to add item to different dropdownlist by passing different parameter?