diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 6b045afea7f912..1b3df3a6eabd3e 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -9,7 +9,7 @@ on:
       version:
         description: 'Version of the Ruby package to release'
         required: true
-        default: '3.3.4'
+        default: '4.0.0'
 
 permissions:
   contents: read
diff --git a/box.c b/box.c
index 9c85426b8c51b8..b55e8bb137a576 100644
--- a/box.c
+++ b/box.c
@@ -234,7 +234,7 @@ rb_box_entry_mark(void *ptr)
     rb_gc_mark(box->ruby_dln_libmap);
     rb_gc_mark(box->gvar_tbl);
     if (box->classext_cow_classes) {
-        rb_mark_tbl(box->classext_cow_classes);
+        rb_mark_set(box->classext_cow_classes);
     }
 }
 
@@ -271,10 +271,10 @@ free_box_st_tables(void *ptr)
 }
 
 static int
-free_classext_for_box(st_data_t _key, st_data_t obj_value, st_data_t box_arg)
+free_classext_for_box(st_data_t key, st_data_t _value, st_data_t box_arg)
 {
     rb_classext_t *ext;
-    VALUE obj = (VALUE)obj_value;
+    VALUE obj = (VALUE)key;
     const rb_box_t *box = (const rb_box_t *)box_arg;
 
     if (RB_TYPE_P(obj, T_CLASS) || RB_TYPE_P(obj, T_MODULE)) {
diff --git a/class.c b/class.c
index 91bea44cd93069..11cdd9a81ddc12 100644
--- a/class.c
+++ b/class.c
@@ -89,8 +89,7 @@ rb_class_unlink_classext(VALUE klass, const rb_box_t *box)
 {
     st_data_t ext;
     st_data_t key = (st_data_t)box->box_object;
-    VALUE obj_id = rb_obj_id(klass);
-    st_delete(box->classext_cow_classes, &obj_id, 0);
+    st_delete(box->classext_cow_classes, &klass, 0);
     st_delete(RCLASS_CLASSEXT_TBL(klass), &key, &ext);
     return (rb_classext_t *)ext;
 }
@@ -203,7 +202,7 @@ rb_class_set_box_classext(VALUE obj, const rb_box_t *box, rb_classext_t *ext)
     // (e.g. st_insert below) that could trigger GC.
     rb_gc_writebarrier_remember(obj);
 
-    st_insert(box->classext_cow_classes, (st_data_t)rb_obj_id(obj), obj);
+    st_insert(box->classext_cow_classes, (st_data_t)obj, 0);
 }
 
 RUBY_EXTERN rb_serial_t ruby_vm_global_cvar_state;