Does not fetch mutliple objects in Set - com.sun.jdi.InvocationException


The mapping I have used is
<set name="usmlCollection" table="dtas_case">

<key column="case_id" not-null="true"/>
<element type="java.lang.String"

formula= "(select distinct cmc.category_code from commodity_category cmc
where cmc.category_id in(
select cms.category_id from commodity_subcategory cms
where cms.subcategory_id in(
select cmm.subcategory_id from commodity_matrix cmm
where cmm.item_id in(
select cm.commodity_item from commodity cm
where cm.case_id = case_id)

)
)
)"

/>

</set>
----------------
when the result set contains just one element I get the resultset with 1 element in the array; However for cases where more than 1 value is present , I get the com.sun.jdi.InvocationException

Can somebody please help or tell me some alternatives ? I cannot change any other hbm mapping for other classes