The source seems to be sxpath.scm -- see "yikes" error which triggers when n is -2 -4 -6 etc. I don't know how to build guile from sources and cannot debug further.
(define (node-pos n)
(lambda (nodeset)
(cond
((not (nodeset? nodeset)) '())
((null? nodeset) nodeset)
((eqv? n 1) (list (car nodeset)))
((negative? n) ((node-pos (+ n 1 (length nodeset))) nodeset))
(else
(or (positive? n) (error "yikes!"))
((node-pos (1- n)) (cdr nodeset))))))
