checkbox problem

Question

i m facing problem Specified cast is not valid.

 

when i use checkbox in gridview

 

my code is here..

<asp:GridView ID="GridView3" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:CheckBox ID="CheckBox1" runat="server" Checked='<%#Eval("active")%>' Text='<%#Eval("id")%>' OnCheckedChanged="CheckBox1_CheckedChanged" AutoPostBack="true" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="name" HeaderText="Name" />
<asp:BoundField DataField="address" HeaderText="Address" />
<asp:BoundField DataField="city" HeaderText="City" />
</Columns>
</asp:GridView>

protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
{
CheckBox chk = (CheckBox)sender;
string st = "0";
if (chk.Checked)
{
st = "1";
}
nc.id = chk.Text;
nc.mode = "status";
nc.active = chk.ToString();
int eff = nc.manipulatNews(nc);
if (eff > 0)
{
fil_grid();
}
}

plz help me

thank you