diff --git a/syndicate/skeleton.rkt b/syndicate/skeleton.rkt
index 4a6b8cf..da4bf99 100644
--- a/syndicate/skeleton.rkt
+++ b/syndicate/skeleton.rkt
@@ -345,13 +345,15 @@
             [(key (in-list path))]
     (step-term term key)))
 
+(define (index? k) (exact-nonnegative-integer? k))
+
 (define (step-term term key)
   (cond [(non-object-struct? term)
          (define v (struct->vector term))
          (define k (+ key 1)) ;; skip the label
-         (if (>= k (vector-length v)) (void) (vector-ref v k))]
+         (when (and (index? k) (< k (vector-length v))) (vector-ref v k))]
         [(list? term)
-         (if (>= key (length term)) (void) (list-ref term key))]
+         (when (and (index? key) (< key (length term))) (list-ref term key))]
         [(hash? term)
          (hash-ref term key (void))]
         [else