Checkbox must be placed within the form tag with runat=server

I have the belowe code, when I load the page it says:

"Checkbox must be placed within the form tag with runat=server"

Why is it throwing this error while the checkbox is inside the form tag?!.

<form id="form1" runat="server">
  <asp:scriptmanager runat="server"></asp:scriptmanager>
       <asp:CheckBox ID="CheckBox1" runat="server" />
<div>

      <rsweb:ReportViewer ID="ReportViewer1" runat="server" Width="900" Height="600"></rsweb:ReportViewer>
</div>
</form>