Setting the default value of a list box

[aaronjohn]

I tried this:
IHTMLElementCollection optionElements = (IHTMLElementCollection) htmlDoc.all.tags("select");
IHTMLOptionElement billingSystem = (IHTMLOptionElement) optionElements.item("billing_system",0);

if( billingSystem != null )
{ billingSystem.value = BillingSystem.ToString();
}

But it throws an exception, saying that optionElements.item returns null.

I also tried this:
IHTMLSelectElement selectElement = (IHTMLSelectElement) htmlDoc.all.tags("select");
IHTMLOptionElement billingSystem = (IHTMLOptionElement) selectElement.item("billing_system",0);

But there was the same problem. Anybody know how to do it properly?