Index: clos.lisp
===================================================================
--- clos.lisp	(revision 12922)
+++ clos.lisp	(working copy)
@@ -2258,6 +2258,8 @@
     (error 'program-error :format-control "Odd number of keyword arguments."))
   (unless (class-finalized-p class)
     (std-finalize-inheritance class))
+  (unless (class-finalized-p class)
+    (error "Can't instantiate unfinalized class ~A." class))
   (let ((class-default-initargs (class-default-initargs class)))
     (when class-default-initargs
       (let ((default-initargs '()))