diff --git a/src/class.c b/src/class.c
index c849eee92..a07cdbce5 100644
--- a/src/class.c
+++ b/src/class.c
@@ -3028,15 +3028,15 @@ mrb_init_class(mrb_state *mrb)
   mrb_class_name_class(mrb, NULL, cls, MRB_SYM(Class));  /* 15.2.3 */
 
   MRB_SET_INSTANCE_TT(cls, MRB_TT_CLASS);
-  mrb_define_method_id(mrb, bob, MRB_SYM(initialize),              mrb_do_nothing,           MRB_ARGS_NONE());
-  mrb_define_method_id(mrb, bob, MRB_OPSYM(not),                   mrb_bob_not,              MRB_ARGS_NONE());
-  mrb_define_method_id(mrb, bob, MRB_OPSYM(eq),                    mrb_obj_equal_m,          MRB_ARGS_REQ(1)); /* 15.3.1.3.1  */
-  mrb_define_method_id(mrb, bob, MRB_SYM(__id__),                  mrb_obj_id_m,             MRB_ARGS_NONE()); /* 15.3.1.3.4  */
-  mrb_define_method_id(mrb, bob, MRB_SYM(__send__),                mrb_f_send,               MRB_ARGS_REQ(1)|MRB_ARGS_REST()|MRB_ARGS_BLOCK());  /* 15.3.1.3.5  */
-  mrb_define_method_id(mrb, bob, MRB_SYM_Q(equal),                 mrb_obj_equal_m,          MRB_ARGS_REQ(1)); /* 15.3.1.3.11 */
-  mrb_define_method_id(mrb, bob, MRB_SYM(instance_eval),           mrb_obj_instance_eval,    MRB_ARGS_OPT(1)|MRB_ARGS_BLOCK());  /* 15.3.1.3.18 */
-  mrb_define_method_id(mrb, bob, MRB_SYM(singleton_method_added),  mrb_do_nothing,           MRB_ARGS_REQ(1));
-  mrb_define_method_id(mrb, bob, MRB_SYM(method_missing),          mrb_obj_missing,          MRB_ARGS_ANY());  /* 15.3.1.3.30 */
+  mrb_define_private_method_id(mrb, bob, MRB_SYM(initialize),              mrb_do_nothing,           MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, bob, MRB_OPSYM(not),                           mrb_bob_not,              MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, bob, MRB_OPSYM(eq),                            mrb_obj_equal_m,          MRB_ARGS_REQ(1)); /* 15.3.1.3.1  */
+  mrb_define_method_id(mrb, bob, MRB_SYM(__id__),                          mrb_obj_id_m,             MRB_ARGS_NONE()); /* 15.3.1.3.4  */
+  mrb_define_method_id(mrb, bob, MRB_SYM(__send__),                        mrb_f_send,               MRB_ARGS_REQ(1)|MRB_ARGS_REST()|MRB_ARGS_BLOCK());  /* 15.3.1.3.5  */
+  mrb_define_method_id(mrb, bob, MRB_SYM_Q(equal),                         mrb_obj_equal_m,          MRB_ARGS_REQ(1)); /* 15.3.1.3.11 */
+  mrb_define_method_id(mrb, bob, MRB_SYM(instance_eval),                   mrb_obj_instance_eval,    MRB_ARGS_OPT(1)|MRB_ARGS_BLOCK());  /* 15.3.1.3.18 */
+  mrb_define_private_method_id(mrb, bob, MRB_SYM(singleton_method_added),  mrb_do_nothing,           MRB_ARGS_REQ(1));
+  mrb_define_private_method_id(mrb, bob, MRB_SYM(method_missing),          mrb_obj_missing,          MRB_ARGS_ANY());  /* 15.3.1.3.30 */
 
   mrb_define_class_method_id(mrb, cls, MRB_SYM(new),               mrb_class_new_class,      MRB_ARGS_OPT(1)|MRB_ARGS_BLOCK());
   mrb_define_method_id(mrb, cls, MRB_SYM(allocate),                mrb_instance_alloc,       MRB_ARGS_NONE());