Form with ListSelect, converter question

I have a form with a ListSelect. To the list select I add a number of Strings. The form works in that I can select multiple items from the list and get a set of strings. I get an error about conversion that I can’t get around. The error states it can’t convert a LinkedHashSet to a String. I tried creating a convertor but that is not working because I can’t create a LinkedHashSet from a String either.

If I remove the form part and just use a panel witth a list select the error never occurs. Is this a form issue?

G Ricker