Update Combobox ItemsSource

I have two combo boxes: cboContinents and cboCountries.

In cboContinents_ValueChanged event, I have: 

selectedContinent = cboContinents.Text.ToString();

List<string>lstCountries = MyClass.GetListOfCountries(selectedContinent);

cboCountries.ItemsSource = lstCountries;

The above logic seems to be working okay as the drop-down list associated with cboCountries is updated. HOWEVER, when an item in cboCountries is selected and I select another control, the text in cboCountries is automatically replaced by another item. It's like the index of the item is not matched with the value..

Pls help.