Unexpected error

Hello,

just trying acl 8.0 express.

With the following code:

(<-- (fa-rete ?l ?r ?v)
      (append ?l1 ?l2 ?l)
      (not (== ?l1 ()))
      (not (== ?l2 ()))
      (fa-rete ?l1 ?r1 ?v1)
      (fa-rete ?l2 ?r2 ?v2)
      (or (and (= ?r (ser ?r1 ?r2))
	        (is ?v (+ ?v1 ?v2)))
	   (and (= ?r (par ?r1 ?r2))
	        (is ?v (/ (+ (/ ?v1) (/ ?v2)))))))
(<- (fa-rete (?q) ?q ?q))

(defun ohm (resistori &optional (val-min 1d300))
   (let ((rete-min))
     (prolog (lisp ?lista resistori)
             (and (fa-rete ?lista ?rete ?valore)
                  (lisp* (let ((val (abs (- ?valore pi))))
                           (when (< val val-min)
                             (setf val-min val
                                   rete-min ?rete))))))
     (values val-min rete-min)))

calling (ohm '(1 2 3 4 5 6 7 8 9 10)) most of the times works as intended.

However, it happened 2 times (out of about 10) that it signalled the
following errors:

Error: `#<unknown object of type number 2 @ #x20ebb952>' is not of the
        expected type `NUMBER'
   [condition type: TYPE-ERROR]


Error: `#<unknown object of type number 26 @ #x20eb4972>' is not of the
        expected type `NUMBER'
   [condition type: TYPE-ERROR]


(lisp-implementation-version) ==> "8.0 [Windows] (Jun 9, 2006 4:01)"

Cheers
P.