bug in retrieve-partial

I have a case where retrieve all works, but retrieve-partial best
crashes.  Here's an example, and a backtrace.  Please let me know if
you need more bug information:


STELLA(77): (retrieve all ?x (info-source ?x tribe-b))
There are 2 solutions:
  #1: ?X=MADONNA-LYMON
  #2: ?X=HAROLD-CREEK
STELLA(78): (retrieve-partial best ?x (info-source ?x tribe-b))
Error: No methods applicable for generic function #<CL:STANDARD-GENERIC-FUNCTION %POSITIVE-SCORE> with args (NULL)
       of classes (CL:SYMBOL)
  [condition type: PROGRAM-ERROR]

Restart actions (select using :continue):
 0: Try calling it again
 1: Return to Top Level (an "abort" restart).
 2: Abort entirely from this (lisp) process.
[1c] STELLA(79): :bt
Evaluation stack:

(CL:METHOD CL:NO-APPLICABLE-METHOD (CL:T)) <-
  (CL:METHOD CONTINUE-PARTIAL-ANTECEDENTS-PROOF (INCREMENTAL-PARTIAL-MATCH CL:T)) <-
  RESUME-PROOF-STRATEGY-AFTER-SUBGOAL <- CONTINUE-STRATEGIES-PROOFS <-
  (CL:METHOD CONTINUE-PARTIAL-STRATEGIES-PROOFS (INCREMENTAL-PARTIAL-MATCH CL:T)) <- EVALUATE-NEXT-MOVE <-
  EXECUTE-BACKWARD-CHAINING-PROOF? <- (CL:METHOD NEXT? (QUERY-ITERATOR)) <- [... NEXT? ] <-
  (CL:METHOD RETRIEVE-PARTIAL-SOLUTIONS (INCREMENTAL-PARTIAL-MATCH CL:T)) <- CALL-RETRIEVE-PARTIAL <-
  %RETRIEVE-PARTIAL <- [... EXCL::%EVAL ] <- CL:EVAL <- TPL:TOP-LEVEL-READ-EVAL-PRINT-LOOP <-
  TPL:START-INTERACTIVE-TOP-LEVEL

Thanks,
R