diff --git a/wip-duuqnd/instruction.lisp b/wip-duuqnd/instruction.lisp
index ba20b59..900e103 100644
--- a/wip-duuqnd/instruction.lisp
+++ b/wip-duuqnd/instruction.lisp
@@ -13,6 +13,11 @@
 (defclass immediate-mixin () ())
 (defclass implied-mixin () ())
 (defclass zero-page-mixin () ())
+
+(defmethod shared-initialize :after ((instance zero-page-mixin) slot-names &rest initargs &key &allow-other-keys)
+  (declare (ignore slot-names initargs))
+  (assert (< (operand instance) #x100)))
+
 (defclass zero-page-x-mixin (zero-page-mixin) ())
 (defclass absolute-mixin () ())
 (defclass absolute-x-mixin (absolute-mixin) ())