Index: /trunk/source/lib/macros.lisp
===================================================================
--- /trunk/source/lib/macros.lisp	(revision 13203)
+++ /trunk/source/lib/macros.lisp	(revision 13204)
@@ -1812,4 +1812,6 @@
          (record-function-info ',(maybe-setf-function-name name)
                                ',(multiple-value-bind (bits keyvect) (encode-lambda-list lambda-list t)
+                                   (unless bits ;; verify failed
+                                     (signal-program-error "Invalid lambda list ~s" (car args)))
                                    (%cons-def-info 'defmethod bits keyvect nil specializers qualifiers))
                                ,env))