Je ne peux pas telecharger des lecons

Bonjour,
bonjour a tous
je vais essayer d expliquer mon probleme
habitant la chine j ai achete un logiciel en chine pour apprendre le chinois, j ai avec mon logiciel deux cartes permettant d acheter pour 110dollard des lecons sur leur site internet
lorsque j essaye d acheter puis de telecharger les lecons apres bien sur m etre identifier le site affiche une page d erreur
la voicie, je ne sais pas quoi, j ai contacter la hotline il me dise que logiquement il n y a pas de pas probleme
merci de m aider
Server Error in '/' Application.
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'ddlcountries' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentOutOfRangeException: 'ddlcountries' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

Source Error:


Line 94:
Line 95: }
Line 96: ddlcountries.SelectedValue = Session["Country"].ToString();
Line 97: ShoppingCart cart = (ShoppingCart)Session["ShoppingCartSession"];
Line 98: Debug.Assert(cart != null);


Source File: f:\ezsino.com\www.ezsino.com\BackUser\Payment.aspx.cs Line: 96

Stack Trace:


[ArgumentOutOfRangeException: 'ddlcountries' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value]
System.Web.UI.WebControls.ListControl.set_SelectedValue(String value) +1776538
UniteSoft.Web.Payment.Page_Load(Object sender, EventArgs e) in f:\ezsino.com\www.ezsino.com\BackUser\Payment.aspx.cs:96
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +34
System.Web.UI.Control.OnLoad(EventArgs e) +99
UniteSoft.Web.PageBase.OnLoad(EventArgs e) in f:\ezsino.com\www.ezsino.com\App_Code\PageBase.cs:35
System.Web.UI.Control.LoadRecursive() +47
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1061




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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.42


est ce que vous pouvez me dire cette page est le fruit de ma maladresse ou de l incompetence du site internet de la societe d editeur
merci de votre aide