Having two classes: (defclass slot-d-1 (mop:standard-direct-slot-definition) ()) (defclass slot-d-2 (mop:standard-direct-slot-definition) (a)) (slot-boundp (make-instance 'slot-d-2) 'sys::initargs) => T (slot-boundp (make-instance 'slot-d-2) 'sys::initargs) => NIL So, having a slot makes initargs unbound. Below is the actual code which suffers from it: (defpackage test (:use :cl :mop)) (in-package test) (defclass storable-class (standard-class) ()) (defmethod validate-superclass ((class standard-class) (superclass storable-class)) t) (defmethod validate-superclass ((class storable-class) (superclass standard-class)) t) (defclass storable-direct-slot-definition (standard-direct-slot-definition) ((storep :initarg :storep :initform t :reader store-slot-p))) (defmethod direct-slot-definition-class ((class storable-class) &key) (find-class 'storable-direct-slot-definition)) ;;; (defclass identifiable () ((id :accessor id :initform nil :storep nil)) (:metaclass storable-class)) The last form receives an error: The slot INITARGS is unbound in the object #<STORABLE-DIRECT-SLOT-DEFINITION {34C1EFE7}>. -- With best regards, Stas.