Iterating Outlook Contacts

Hi there!, I have this very strange problem:

I want to iterate through Outlook contacts located in a public folder, but iteration stops randomly giving different error codes, among them these ones: -833601531, -695189499, -625983483, -37732347. Everytime I get the same text message: "The operation failed".

for (int i = 1; i <= mexicoContacts.Items.Count; i++)
{

if (mexicoContacts.Items[i] is Outlook.ContactItem)
{

Outlook.ContactItem contact = (Outlook.ContactItem)mexicoContacts.Items[i];
if (contact.LastName != null)
listBox1.Items.Add(contact.LastName);

}

Any Ideas?

Thanks...