rthm-seq-bar::get-nth-event: index = -1!

I’m having weirdness with sc-force-rest2, which throws the error above when I call it from this loop

 (loop
    ;;get instruments
    for ins in '(p1 p2 p3 p4 p5)
    ;;different numbers of cycles over activity curve for each part
    for cycles in '(1 2 13 2 15)
    ;;get total notes for scaling activity curve
    for notes =  (total-notes (get-data ins (ensemble opus)))
     do
       (next-event opus ins t 1);start slurping events
       (loop
	  for ne = (next-event opus ins t)
	  for i from 1
	  ;;scaled activity envelope 
	  with  activity = (make-ale (genal) (floor notes cycles))
	  while ne
	  do 
	    (when (not (active activity))
	      (sc-force-rest2 opus (bar-num ne) (bar-pos ne) ins)))))

Backtrace:
0: ((:METHOD GET-NTH-EVENT (T RTHM-SEQ-BAR)) -1 ..) [fast-method]
1: ((:METHOD SC-FORCE-REST2 (SLIPPERY-CHICKEN T T T)) ..) [fast-method]