Dynamically Loaded Select Boxes

Each time I dynamically load the options of a select box and then try to do anything with it (such as clicking it or using other JS code on it) I get the following Firefox error:Error: Index or size is negative or greater than the allowed amount = NS_ERROR_DOM_INDEX_SIZE_ERR

I am using the following code:<select id="day" name="day"></select>
<script language="javascript">
var options_list = new Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31);
selectbox_element = document.getElementById('day');
selectbox_element.options.length = 0; // Remove any previous values.

for (var i = 0; i < options_list.length; i++) {
selectbox_element.options[i] = new Option(options_list[i], options_list[i]);
}
</script>

The "options_list" array is filled beforehand using a simple PHP for() loop.

If I create the exact same options but do so by writing them into the HTML using PHP, then I don't see the error message anymore.

This problem happens in both Firefox 1.5 and Internet Explorer.