diff --git a/internal/namespace.h b/internal/namespace.h
index c68f0987aa..9ffc9a5c8b 100644
--- a/internal/namespace.h
+++ b/internal/namespace.h
@@ -40,7 +40,7 @@ struct rb_namespace_struct {
 };
 typedef struct rb_namespace_struct rb_namespace_t;
 
-#define NAMESPACE_OBJ_P(obj) (CLASS_OF(obj) == rb_cNamespace)
+#define NAMESPACE_OBJ_P(obj) (rb_obj_class(obj) == rb_cNamespace)
 
 #define NAMESPACE_ROOT_P(ns) (ns && !ns->is_user)
 #define NAMESPACE_USER_P(ns) (ns && ns->is_user)
diff --git a/namespace.c b/namespace.c
index 81017f902b..8d50ba4415 100644
--- a/namespace.c
+++ b/namespace.c
@@ -877,7 +877,8 @@ rb_initialize_main_namespace(void)
         namespace_experimental_warned = 1;
     }
 
-    main_ns = rb_class_new_instance_pass_kw(0, NULL, rb_cNamespace);
+    main_ns = rb_class_new_instance(0, NULL, rb_cNamespace);
+    VM_ASSERT(NAMESPACE_OBJ_P(main_ns));
     ns = rb_get_namespace_t(main_ns);
     ns->ns_object = main_ns;
     ns->ns_id = namespace_generate_id();