diff --git a/libguile/dynwind.c b/libguile/dynwind.c
index 304ba7711..3108f17c6 100644
--- a/libguile/dynwind.c
+++ b/libguile/dynwind.c
@@ -186,7 +186,8 @@ scm_swap_bindings (SCM glocs, SCM vals)
   while (SCM_NIMP (vals))
     {
       tmp = SCM_GLOC_VAL (SCM_CAR (glocs));
-      SCM_SETCDR (SCM_CAR (glocs) - 1L, SCM_CAR (vals));
+      SCM_SETCDR (SCM_PACK (SCM_UNPACK (SCM_CAR (glocs)) - 1L),
+		  SCM_CAR (vals));
       SCM_SETCAR (vals, tmp);
       glocs = SCM_CDR (glocs);
       vals = SCM_CDR (vals);