CustomValidator Does Not Work

<asp:TextBox ID="TextBox18" runat="server"Text="></asp:TextBox>
    <br of >       
    <asp:CustomValidator ID="CustomValidator1" runat="server"
         ErrorMessage="No asp.net" ControlToValidate="TextBox18" OnServerValidate="CustomValidator1_ServerValidate">/asp:CustomValidator><br/>
    <asp:Button ID="Button5" runat="server" Text="Send"/>

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgsargs)
    {
        var text = args.Value;
        if(text.IndexOf("asp.net")>=0)
        {
            args.IsValid=true;
        }
        else
        {
            args.IsValid=false;
        }
    }