Hvorfor denne fejl ?

An unhandled exception of type 'System.InvalidCastException' occurred in unread_mail_c.exe

Additional information: Specified cast is not valid.

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Den opstår i denne linje "item = (Outlook.MailItem)lists.Items[i];" i for løkken


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do
{
    Outlook.Application oApp = new Outlook.Application();
    Outlook.NameSpace oNameSpace= oApp.GetNamespace("MAPI");
    oNameSpace.Logon(Missing.Value,Missing.Value,false,true);
    Outlook.MAPIFolder lists = oNameSpace.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderInbox );
    Outlook.MailItem item;
    unread = 0;
    for(int i=1; i<lists.Items.Count; i++)
    {
        item = (Outlook.MailItem)lists.Items[i];
        if(item.UnRead) unread++;
    }
    Console.WriteLine("Antal ulæste beskeder = "+unread);


    F.Pulse_reset(adresse);
    F.wait_1sek();
    F.Pulse_mail(adresse, unread);

    for (int i = 0 ; i < tid_check ; i++ )
        F.wait_1sek();

}while (true);