diff --git a/module/ice-9/psyntax.scm b/module/ice-9/psyntax.scm
index bd4bd6723..054d21795 100644
--- a/module/ice-9/psyntax.scm
+++ b/module/ice-9/psyntax.scm
@@ -744,18 +744,19 @@
                             (search-list-rib sym subst marks symnames fst mod))))))))
         (define search-list-rib
           (lambda (sym subst marks symnames ribcage mod)
-            (let f ((symnames symnames) (i 0))
+            (let f ((symnames symnames)
+                    (rlabels (ribcage-labels ribcage))
+                    (rmarks (ribcage-marks ribcage)))
               (cond
                ((null? symnames) (search sym (cdr subst) marks mod))
-               ((and (eq? (car symnames) sym)
-                     (same-marks? marks (list-ref (ribcage-marks ribcage) i)))
-                (let ((n (list-ref (ribcage-labels ribcage) i)))
+               ((and (eq? (car symnames) sym) (same-marks? marks (car rmarks)))
+                (let ((n (car rlabels)))
                   (if (pair? n)
                       (if (equal? mod (car n))
                           (values (cdr n) marks)
-                          (f (cdr symnames) (fx+ i 1)))
+                          (f (cdr symnames) (cdr rlabels) (cdr rmarks)))
                       (values n marks))))
-               (else (f (cdr symnames) (fx+ i 1)))))))
+               (else (f (cdr symnames) (cdr rlabels) (cdr rmarks)))))))
         (define search-vector-rib
           (lambda (sym subst marks symnames ribcage mod)
             (let ((n (vector-length symnames)))