JessRuleEngine : Undefining accumulate

Question

I want to count elements in a list that have the same ID

(defrule UniqueIdentifier_testfile
   (P (ID_Jess ?id_context) (k $?ass))

   ?c <- (accumulate (bind ?count 0)                        ;; initializer
            (bind ?count (+ ?count 1))                    ;; action
            ?count                                        ;; result
            (and 
               (P (ID_Jess ?id_as1&:(member$ ?id_as1 ?ass)) (id ?id_ref1))
               (P (ID_Jess ?id_as2&:(member$ ?id_as2 ?ass)) (id ?id_ref2))
               (and (neq ?id_as1 ?id_as2) (eq ?id_ref1 ?id_ref2))
            )
        ) ;; CE

   (test (neq ?c 0))
 => 
   (printout t "UniqueIdentifier_testfile -->  FIRE ! (" ?id_context ":P).id :{"?ass"} -> 'id' not uniques" crlf)
)

As you can see : a fact of type P has a "real" ID which is reified in the slot "ID_Jess" and an other slot "id" which I'd like to test if it is unique too. In modelling words :

(deftemplate P 
  (slot ID_Jess)
  (slot id )
  (slot m )
  (multislot k )
)

It all looks fine, and "compile" allright. But the execution gives me that error :

    Jess reported an error in routine HasLHS.addPattern.
  Message: Variable used before definition: c.
  Program text: ( defrule UniqueIdentifier [...])  at line 70 in file <eval pipe>.
    at jess.HasLHS.a(Unknown Source)
    ...

Anybody got a clue why that ?c is not defined after the accumulate has been executed ? Even if the condition is never fulfilled, the counter (?count) which is binded as result will at least be a defined '0'.. Am I wrong ? If I specify an easier conditional element, the accumulate function executes as expected..