ASP.Net DropDownList has a SelectedValue which is invalid because it does not exist in the list

Hi, Before this, everything is fine when running the project After I've put "SelectedValue" below to the field,

    <asp:TemplateField HeaderText="Interest">
        <ItemTemplate>
            <asp:DropDownList ID="ddl" OnSelectedIndexChanged="ddl_IndexChanged" SelectedValue='<%# bind("int_id") %>' AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </ItemTemplate>
        <EditItemTemplate>
            <asp:DropDownList ID="ddl" OnSelectedIndexChanged="ddl_IndexChanged" SelectedValue='<%# bind("int_id") %>' AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </EditItemTemplate>
        <FooterTemplate>
            <asp:DropDownList ID="ddl" OnSelectedIndexChanged="ddl_IndexChanged" SelectedValue='<%# bind("int_id") %>' AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </FooterTemplate>
    </asp:TemplateField>

I then have got this exception message 'ddl' has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: valueSystem.Web when it is to call this event

        private void BindData()
        {
            int userid;
            bool b = int.TryParse(lb_userid.Text, out userid);
            DataTable table = new DataTable();
            using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Mssqlconn2"].ConnectionString))
            {
                string sql = "Select * from student where teacher_id=@teacher_id";
                //string sql = "Select * from student";
                using (SqlCommand cmd = new SqlCommand(sql, conn))
                {
                    using (SqlDataAdapter ad = new SqlDataAdapter(cmd))
                    {
                        cmd.Parameters.Add("@teacher_id", SqlDbType.Int).Value = userid;
                        ad.Fill(table);
                    }
                }
            }
            GridView1.DataSource = table;
            GridView1.DataBind();
        }

Here are other relevant events.

        protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                if (e.Row.RowState == DataControlRowState.Edit)
                {
                    DropDownList ddl = (DropDownList)e.Row.FindControl("ddl");
                    this.BindDropDown(ddl);
                }
                else
                {
                    DropDownList ddl = (DropDownList)e.Row.FindControl("ddl");
                    this.BindDropDown(ddl);
                }
            }

            if (e.Row.RowType == DataControlRowType.Footer)
            {
                DropDownList ddl = (DropDownList)e.Row.FindControl("ddl");
                this.BindDropDown(ddl);
            }            
        }

        private void BindDropDown(DropDownList para_ddl)
        {
            SqlCommand cmd = new SqlCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "select stu_id,rtrim(ltrim(cast(stu_id as varchar)))+' '+stu_name stu_name from student where stu_id>=12 order by 1";
            para_ddl.DataSource = GetData(cmd);
            para_ddl.DataTextField = "stu_name";
            para_ddl.DataValueField = "stu_id";
            para_ddl.DataBind();
            para_ddl.Items.Insert(0, new ListItem("Please select"));
        }