Did some further testing and found one interesting piece of data. If I select any value in the select list other than the first, everything is fine (that is to say, the output XML looks like <Acts>Some value</Acts> and my process works. However, when I select the first item, then the situation is as described earlier - <Acts> is empty, the process fails. It's almost as though the combo box doesn't realize that a choice has been made.

I will try the hack of putting in a dummy first element (e.g. "Select One...") and see if that provides a workaround.

Enjoy,
RB