Issues with adding ListBox item from modal dialog

[Kyrus]

I've been googling for the last 2 hours attempting to find a solution to my question, which is this:

What I have a checkbox list in a modal dialog, what I want to happen is when one of the checkboxs are click it adds the value to a listbox in the parent page.

I've tried the following and it doesnt seem to work:

var lblUsers = window.dialogArguments.Form1.lblUsers;
var opt = new Option(Text, Value);
lblUsers.options[lblUsers.options.length] = opt;

I'm getting an "Object doesn't support this property or method" error back from javascript.

I can edit existing options, I just can't seem to add new ones.

Any ideas?