Index: lib/sequences.lisp
===================================================================
--- lib/sequences.lisp	(revision 10579)
+++ lib/sequences.lisp	(working copy)
@@ -469,6 +469,8 @@
         (cond ((null (setq seq (%car p))) (return))
               ((consp seq)
                (%rplaca p (%cdr seq))
+               (unless (listp (%car p))
+                 (report-bad-arg (%car p) 'list))
                (%rplaca args (%car seq)))
               ((eq index (length seq)) (return))
               (t (%rplaca args (elt seq index))))