Tried to bind * to NULL value.

I got "WARNING: Tried to bind |V|?s2 to NULL value.  Potentially a PowerLoom bug". I can't see any error in my code and the negative questio works. 
Thanks.

;;;;
;;;; PowerLoom 4.0.0.beta
;;;;

(in-package "STELLA")

(defmodule "TEST"
  :documentation "Test module"
  :includes ("PL-USER"))

(in-module "TEST")

(set-feature justifications)

(defrelation individual (?x))
(defrelation bounded (?x))
(defrelation unbounded (?x))
(defrelation member (?x ?s))
(defrelation equal (?x ?y))

(assert (forall (?x) (or (set ?x) (individual ?x))))
(assert (forall (?x) (or (not (set ?x)) (not (individual ?x)))))

(assert (forall (?x) (or (bounded ?x) (unbounded ?x))))
(assert (forall (?x) (or (not (bounded ?x)) (not (unbounded ?x)))))

(assert (forall (?x ?s) (=> (member ?x ?s) (bounded ?x))))
(assert (forall (?x ?s) (=> (member ?x ?s) (set ?s))))

(assert (forall (?x ?y) (=> (equal ?x ?y) (equal ?y ?x))))

(assert (forall (?s1 ?s2) (=> (and (set ?s1) (set ?s2))
           (<=> (forall (?x) (<=> (member ?x ?s1) (member ?x ?s2))) 
              (equal ?s1 ?s2)))))

              
(assert (set A))
(assert (set B))
(assert (individual C))
(assert (member C A))
(assert (forall (?m) (=> (member ?m A) (member ?m B))))
(assert (forall (?m) (=> (member ?m B) (member ?m A))))

;;;;
;;;;  ERROR?
;;;;

* (ask (equal A B))
WARNING: Tried to bind |V|?s2 to NULL value.  Potentially a PowerLoom bug

debugger invoked on a COMMON-LISP:SIMPLE-ERROR:
  There is no applicable method for the generic function
    #<COMMON-LISP:STANDARD-GENERIC-FUNCTION COPY (13)>
  when called with arguments
    (:NULL_VALUE).

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

((SB-PCL::FAST-METHOD COMMON-LISP:NO-APPLICABLE-METHOD (COMMON-LISP:T))
 #<unavailable argument>
 #<unavailable argument>
 #<COMMON-LISP:STANDARD-GENERIC-FUNCTION COPY (13)>)[:EXTERNAL]
0] 0

* (ask (not (equal A B)))

FALSE
*
* (why)
1 (NOT (EQUAL A B))
    follows by Modus Ponens
    with substitution {?x/C, ?x/, ?x/, ?x/, ?s2/B, ?s1/A}
    since 1.1 ! (FORALL (?x1 ?x2)
                   (<= (NOT (EQUAL ?x1 ?x2))
                       (AND (SET ?x1)
                            (SET ?x2)
                            (OR (EXISTS (?x)
                                   (AND (MEMBER ?x ?x1)
                                        (NOT (MEMBER ?x ?x2))))
                                (EXISTS (?x)
                                   (AND (MEMBER ?x ?x2)
                                        (NOT (MEMBER ?x ?x1))))
                                (AND (NOT (EQUAL ?x1 ?x2))
                                     (FORALL (?x)
                                        (<= (MEMBER ?x ?x2)
                                            (MEMBER ?x ?x1)))
                                     (FORALL (?x)
                                        (<= (MEMBER ?x ?x1)
                                            (MEMBER ?x ?x2))))))))
    and   1.2   (OR (EXISTS (C)
                       (AND (MEMBER C A)
                            (NOT (MEMBER C B))))
                    (EXISTS (?x)
                       (AND (MEMBER ?x B)
                            (NOT (MEMBER ?x A))))
                    (AND (NOT (EQUAL A B))
                         (FORALL (?x)
                            (<= (MEMBER ?x B)
                                (MEMBER ?x A)))
                         (FORALL (?x)
                            (<= (MEMBER ?x A)
                                (MEMBER ?x B)))))
    and   1.3 ! (SET B)
    and   1.4 ! (SET A)

1.2 (OR (EXISTS (C)
           (AND (MEMBER C A)
                (NOT (MEMBER C B))))
        (EXISTS (?x)
           (AND (MEMBER ?x B)
                (NOT (MEMBER ?x A))))
        (AND (NOT (EQUAL A B))
             (FORALL (?x)
                (<= (MEMBER ?x B)
                    (MEMBER ?x A)))
             (FORALL (?x)
                (<= (MEMBER ?x A)
                    (MEMBER ?x B)))))
    follows by Or-Introduction
    since 1.2.1 (EXISTS (C)
                   (AND (MEMBER C A)
                        (NOT (MEMBER C B))))

1.2.1 (EXISTS (C)
         (AND (MEMBER C A)
              (NOT (MEMBER C B))))
    follows by Existential Introduction
    since 1.2.1.1 ! (MEMBER C A)
    and   1.2.1.2   (NOT (MEMBER C B))

|kv|(<|i|@JUSTIFICATION,|i|@EXPLANATION-INFO> <|i|@PRIMITIVE-STRATEGY,|i|@EXPLAN
ATION-INFO> <|i|@JUSTIFICATION,|i|@EXPLANATION-INFO> <|i|@PRIMITIVE-STRATEGY,|i|
@EXPLANATION-INFO> <|i|@PRIMITIVE-STRATEGY,|i|@EXPLANATION-INFO> <|i|@JUSTIFICAT
ION,|i|@EXPLANATION-INFO> <|i|@PRIMITIVE-STRATEGY,|i|@EXPLANATION-INFO> <|i|@JUS
TIFICATION,|i|@EXPLANATION-INFO>)
*
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