[Plugin: Register Plus Redux] AutoComplete Query Select Values

I am trying to use the Autocomplete query with the Register Plus Redux and supply a value for one of my custom fields. The custom field is a Select field of a possible 32 option values. When I pass a value for this field in the URL (&field_name=value), it is ignored and the first option value for the select field appears in the registration form.

I have two other select custom fields and I cannot pre-fill these either.

Are select/option fields handled differently in the query statement? I have no problem with pre-filling the standard text fields.

Thanks.