Javascript problem when setting value and using portal pages

[stancho]

Hi all,

I have combo box prompt (data from datasource).
I change the first element of the combo box with the following javascript:

<script>
document.formWarpRequest._oLstChoicesCountries[0].text = 'Please, select a country';
</script>

The name of the combo box is set to 'Countries'.

It is working if I launch the report alone.
But if I try to add the report in a portal page I receive a javascript error:

'document.formWarpRequest._oLstChoicesBranch1.0' is null or not an object

Any ideas?