```common lisp
(in-package #:sicl-clos)

(defmethod class-prototype :before ((class class))
  (unless (class-finalized-p class)
    (error "the class is not finalized")))
```

Use explicit package marker with IN-PACKAGE.
user
```common lisp
(cl:in-package #:sicl-clos)

(defmethod class-prototype :before ((class class))
  (unless (class-finalized-p class)
    (error "the class is not finalized")))
```
assistant