Tried to bind |V|?x to NULL value.

Hello!

I'm a newbie trying to get a hang of this nice powerloom tool, but
I've run into a problem. During some simple tests, the bug mentioned
in the subject appeared, and I don't know what to make of it. Here's a
transcript of my session, which will explain it better than my own
words:

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PL-USER |= (powerloom-information)

|L|"PowerLoom 3.2.0
STELLA 3.4.0 [JAVA]
java.vendor = Sun Microsystems Inc.
java.version = 1.6.0_06
os.arch = i386
os.name = Linux
os.version = 2.6.25-2-486"

PL-USER |= (defobject alice)

|i|ALICE

PL-USER |= (defobject bob)

|i|BOB

PL-USER |= (defrelation friend (?x ?y))

|r|FRIEND

PL-USER |= (ask (friend alice bob))

UNKNOWN

PL-USER |= (assert (forall (?x ?y) (friend ?x ?y)))

|P|(FORALL (?x ?y)
   (<= (FRIEND ?x ?y)
       TRUE))

PL-USER |= (ask (friend alice bob))

UNKNOWN

PL-USER |= (retrieve (friend ?x ?y))

WARNING: Tried to bind |V|?x to NULL value.  Potentially a PowerLoom bug
No solutions.


PL-USER |= (ask (friend alice bob))

UNKNOWN

PL-USER |= (ask (forall (?x ?y) (friend ?x ?y)))

UNKNOWN

PL-USER |= (all-facts-of alice)

()

PL-USER |= (all-facts-of friend)

(|P|(RELATION FRIEND) |P|(NTH-DOMAIN FRIEND 0 THING) |P|(NTH-DOMAIN
FRIEND 1 THING) |P|(DUPLICATE-FREE FRIEND) |P|(FORALL (?x ?x)
   (<= (FRIEND ?x ?x)
       TRUE)))

PL-USER |= (ask (forall ?x (friend ?x ?x)))

TRUE

PL-USER |=

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I hope someone can help me to work around this. Thanks in advance.