[rules-users] Handle not found for object if using treeset

Hi All, i've a simple rules that retract objects form the WM rule "Retract From ArrayList" when $total: ArrayList() from collect ( PrestationFacturable() ) then retract($total.get(0)); end at the end of fireAll all the fact are well retracted but if I replace ArrayList by TreeSet rule "Retract From TreeSet" when $tree: TreeSet() from collect ( PrestationFacturable() ) then retract($tree.first()); end I got the following exception org.drools.spi.ConsequenceException: org.drools.FactException: Retract error: handle not found for object: [6, val=1]. Is it in the working memory? because the $tree (at second run) still contains the object that as been removed can somebody can explain this behavior ? Thanks in advance -P.