Problem using instance-of

Hello,
I am new to using PowerLoom and have some troubles with and exception 
that is thrown whenever I want to find all instances of one certain type 
of concept. I figured, since it is the only kind of concept for which 
this happens, that it is most probably something in my definition of 
that concept of one of the sub-concepts.


(DEFCONCEPT WIND-SWELL (?S SWELL)
:<=> (AND (HAS-CONDITION ?S ?SP) (SHORT-PERIOD ?SP) ) )

(DEFCONCEPT GROUND-SWELL (?S SWELL)
:<=> (AND (HAS-CONDITION ?S ?LP) (LONG-PERIOD ?LP)) )

(DEFCONCEPT SHORT-PERIOD (?P PERIOD)
:<=> (OR (= (HAS-PERIOD ?P) 7) (< (HAS-PERIOD ?P) 7) ) )

(DEFCONCEPT LONG-PERIOD (?P PERIOD)
:<=> (OR (> (HAS-PERIOD ?P) 13)  (= (HAS-PERIOD ?P) 13) ) )

(DEFCONCEPT PERIOD (SWELL-CONDITION)
:=> (EXISTS ?I (AND (INTEGER ?I) (HAS-PERIOD ?SELF ?I)) ) )

(DEFCONCEPT SWELL (SURF-CONDITION)
:=> (EXISTS ?P (AND (PERIOD ?P) (HAS-CONDITION ?SELF ?P)) )
:=> (EXISTS ?O (AND (ORIGIN ?O) (HAS-PROP ?SELF ?O)) )
:=> (EXISTS ?SD (AND (SWELL-DIRECTION ?SD) (HAS-CONDITION ?SELF ?SD)) )
:=> (EXISTS ?S (AND (SIZE ?S) (HAS-PROP ?SELF ?S)) )
:=> (EXISTS ?E (AND (EFFECT ?E) (HAS-EFFECT ?SELF ?E)) ) )

Those are all the concepts that are involved here and I only have one 
instance in my file. I am still getting used to the LISP way of thinking 
so don't worry hurting my feelings by telling me that what I did there 
is absolute nonsense. This is the exception being thrown:

 >> Error: Invocation Target Exception in funcall of function:
     public static edu.isi.powerloom.logic.QueryIterator 
edu.isi.powerloom.logic.Logic.retrieveEvaluatorWrapper(edu.isi.stella.Cons) 
((ALL (INSTANCE-OF ?S @SWELL)))
     java.lang.ClassCastException: edu.isi.powerloom.logic.Description 
cannot be cast to edu.isi.powerloom.logic.NamedDescription
     edu.isi.powerloom.logic.Description cannot be cast to 
edu.isi.powerloom.logic.NamedDescription
java.lang.ClassCastException: edu.isi.powerloom.logic.Description cannot 
be cast to edu.isi.powerloom.logic.NamedDescription
     at 
edu.isi.powerloom.logic.Proposition.extractCollectionArgument(Proposition.java:3639)
     at 
edu.isi.powerloom.logic.ControlFrame.tryScanCollectionProof(ControlFrame.java:1447)
     at 
edu.isi.powerloom.logic.ControlFrame.executeProofStrategy(ControlFrame.java:2298)
     at 
edu.isi.powerloom.logic.ControlFrame.continueCurrentOrNextStrategy(ControlFrame.java:2501)
     at 
edu.isi.powerloom.logic.ControlFrame.continueStrategiesProofs(ControlFrame.java:2619)
     at 
edu.isi.powerloom.logic.ControlFrame.evaluateNextMove(ControlFrame.java:4526)
     at 
edu.isi.powerloom.logic.QueryIterator.executeBackwardChainingProofP(QueryIterator.java:1878)
     at edu.isi.powerloom.logic.QueryIterator.nextP(QueryIterator.java:1357)
     at edu.isi.powerloom.logic.Logic.callRetrieve(Logic.java:16537)
     at edu.isi.powerloom.logic.Logic.retrieve(Logic.java:16646)
     at 
edu.isi.powerloom.logic.Logic.retrieveEvaluatorWrapper(Logic.java:16654)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
     at 
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
     at java.lang.reflect.Method.invoke(Method.java:597)
     at edu.isi.stella.javalib.Native.funcall(Native.java:608)
     at edu.isi.stella.Cons.evaluateConsTree(Cons.java:8256)
     at edu.isi.stella.Stella_Object.evaluate(Stella_Object.java:4341)
     at edu.isi.powerloom.logic.Logic.evaluateLogicCommand(Logic.java:31393)
     at edu.isi.powerloom.logic.Logic.logicCommandLoop(Logic.java:31292)
     at edu.isi.powerloom.logic.Logic.powerloom(Logic.java:37136)
     at edu.isi.powerloom.PLI.main(PLI.java:4850)

Thanks already,
Kalle