diff --git a/include/mruby/class.h b/include/mruby/class.h
index a3e1c0d1e..1bf71aa6c 100644
--- a/include/mruby/class.h
+++ b/include/mruby/class.h
@@ -57,7 +57,8 @@ mrb_class(mrb_state *mrb, mrb_value v)
    19:   is_prepended
    18:   is_origin
    17:   is_inherited (used by method cache)
-   6-16: unused
+   7-16: unused
+   6:    prohibit Class#allocate
    0-5:  instance type
 */
 #define MRB_FL_CLASS_IS_PREPENDED (1 << 19)
@@ -74,6 +75,9 @@ mrb_class(mrb_state *mrb, mrb_value v)
 #define MRB_INSTANCE_TT_MASK (0x1F)
 #define MRB_SET_INSTANCE_TT(c, tt) ((c)->flags = (((c)->flags & ~MRB_INSTANCE_TT_MASK) | (char)(tt)))
 #define MRB_INSTANCE_TT(c) (enum mrb_vtype)((c)->flags & MRB_INSTANCE_TT_MASK)
+#define MRB_FL_UNDEF_ALLOCATE (1 << 6)
+#define MRB_UNDEF_ALLOCATOR(c) (mrb_assert((c)->tt == MRB_TT_CLASS), (c)->flags |= MRB_FL_UNDEF_ALLOCATE)
+#define MRB_UNDEF_ALLOCATOR_P(c) ((c)->flags & MRB_FL_UNDEF_ALLOCATE)
 
 MRB_API void mrb_define_method_raw(mrb_state*, struct RClass*, mrb_sym, mrb_method_t);
 MRB_API void mrb_alias_method(mrb_state*, struct RClass *c, mrb_sym a, mrb_sym b);
diff --git a/mrbgems/mruby-binding/src/binding.c b/mrbgems/mruby-binding/src/binding.c
index 08e81fb72..c269facfc 100644
--- a/mrbgems/mruby-binding/src/binding.c
+++ b/mrbgems/mruby-binding/src/binding.c
@@ -397,7 +397,8 @@ void
 mrb_mruby_binding_gem_init(mrb_state *mrb)
 {
   struct RClass *binding = mrb_define_class(mrb, "Binding", mrb->object_class);
-  MRB_SET_INSTANCE_TT(binding, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(binding, MRB_TT_OBJECT);
+  MRB_UNDEF_ALLOCATOR(binding);
   mrb_undef_class_method(mrb, binding, "new");
   mrb_undef_class_method(mrb, binding, "allocate");
 
diff --git a/mrbgems/mruby-complex/src/complex.c b/mrbgems/mruby-complex/src/complex.c
index f0e5829d1..a2c579058 100644
--- a/mrbgems/mruby-complex/src/complex.c
+++ b/mrbgems/mruby-complex/src/complex.c
@@ -395,7 +395,8 @@ void mrb_mruby_complex_gem_init(mrb_state *mrb)
   struct RClass *comp;
 
   comp = mrb_define_class_id(mrb, MRB_SYM(Complex), mrb_class_get_id(mrb, MRB_SYM(Numeric)));
-  MRB_SET_INSTANCE_TT(comp, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(comp, MRB_TT_COMPLEX);
+  MRB_UNDEF_ALLOCATOR(comp);
 
   mrb_undef_class_method(mrb, comp, "new");
   mrb_define_class_method(mrb, comp, "rectangular", complex_s_rect, MRB_ARGS_REQ(1)|MRB_ARGS_OPT(1));
diff --git a/mrbgems/mruby-data/src/data.c b/mrbgems/mruby-data/src/data.c
index dd077619c..2fbc51242 100644
--- a/mrbgems/mruby-data/src/data.c
+++ b/mrbgems/mruby-data/src/data.c
@@ -492,7 +492,8 @@ mrb_mruby_data_gem_init(mrb_state* mrb)
 {
   struct RClass *d;
   d = mrb_define_class(mrb, "Data",  mrb->object_class);
-  MRB_SET_INSTANCE_TT(d, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(d, MRB_TT_STRUCT);
+  MRB_UNDEF_ALLOCATOR(d);
 
   mrb_undef_class_method(mrb, d, "new");
   mrb_define_class_method(mrb, d, "define",          mrb_data_s_def,      MRB_ARGS_ANY());
diff --git a/mrbgems/mruby-method/src/method.c b/mrbgems/mruby-method/src/method.c
index deb37b4a5..f12cc44b6 100644
--- a/mrbgems/mruby-method/src/method.c
+++ b/mrbgems/mruby-method/src/method.c
@@ -546,7 +546,8 @@ mrb_mruby_method_gem_init(mrb_state* mrb)
   struct RClass *unbound_method = mrb_define_class_id(mrb, MRB_SYM(UnboundMethod), mrb->object_class);
   struct RClass *method = mrb_define_class_id(mrb, MRB_SYM(Method), mrb->object_class);
 
-  MRB_SET_INSTANCE_TT(unbound_method, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(unbound_method, MRB_TT_OBJECT);
+  MRB_UNDEF_ALLOCATOR(unbound_method);
   mrb_undef_class_method(mrb, unbound_method, "new");
   mrb_define_method(mrb, unbound_method, "bind", unbound_method_bind, MRB_ARGS_REQ(1));
   mrb_define_method(mrb, unbound_method, "super_method", method_super_method, MRB_ARGS_NONE());
@@ -561,7 +562,8 @@ mrb_mruby_method_gem_init(mrb_state* mrb)
   mrb_define_method(mrb, unbound_method, "owner", method_owner, MRB_ARGS_NONE());
   mrb_define_method(mrb, unbound_method, "name", method_name, MRB_ARGS_NONE());
 
-  MRB_SET_INSTANCE_TT(method, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(method, MRB_TT_OBJECT);
+  MRB_UNDEF_ALLOCATOR(method);
   mrb_undef_class_method(mrb, method, "new");
   mrb_define_method(mrb, method, "==", method_eql, MRB_ARGS_REQ(1));
   mrb_define_method(mrb, method, "eql?", method_eql, MRB_ARGS_REQ(1));
diff --git a/mrbgems/mruby-rational/src/rational.c b/mrbgems/mruby-rational/src/rational.c
index 2fadf5d60..b3a5f945c 100644
--- a/mrbgems/mruby-rational/src/rational.c
+++ b/mrbgems/mruby-rational/src/rational.c
@@ -767,7 +767,8 @@ void mrb_mruby_rational_gem_init(mrb_state *mrb)
   struct RClass *rat;
 
   rat = mrb_define_class_id(mrb, MRB_SYM(Rational), mrb_class_get_id(mrb, MRB_SYM(Numeric)));
-  MRB_SET_INSTANCE_TT(rat, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(rat, MRB_TT_RATIONAL);
+  MRB_UNDEF_ALLOCATOR(rat);
   mrb_undef_class_method(mrb, rat, "new");
   mrb_define_class_method(mrb, rat, "_new", rational_s_new, MRB_ARGS_REQ(2));
   mrb_define_method(mrb, rat, "numerator", rational_numerator, MRB_ARGS_NONE());
diff --git a/mrbgems/mruby-struct/src/struct.c b/mrbgems/mruby-struct/src/struct.c
index 4f6dadcfb..f4a5bab9a 100644
--- a/mrbgems/mruby-struct/src/struct.c
+++ b/mrbgems/mruby-struct/src/struct.c
@@ -697,7 +697,8 @@ mrb_mruby_struct_gem_init(mrb_state* mrb)
 {
   struct RClass *st;
   st = mrb_define_class(mrb, "Struct",  mrb->object_class);
-  MRB_SET_INSTANCE_TT(st, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(st, MRB_TT_STRUCT);
+  MRB_UNDEF_ALLOCATOR(st);
 
   mrb_define_class_method(mrb, st, "new",             mrb_struct_s_def,       MRB_ARGS_ANY());  /* 15.2.18.3.1  */
 
diff --git a/src/class.c b/src/class.c
index 8f33cd736..4eac5f240 100644
--- a/src/class.c
+++ b/src/class.c
@@ -1949,10 +1949,10 @@ mrb_instance_alloc(mrb_state *mrb, mrb_value cv)
   else if (ttype == 0) {
     ttype = MRB_TT_OBJECT;
   }
+  if (MRB_UNDEF_ALLOCATOR_P(c)) {
+    mrb_raisef(mrb, E_TYPE_ERROR, "allocator undefined for %v", cv);
+  }
   if (ttype <= MRB_TT_CPTR) {
-    if (ttype == MRB_TT_UNDEF) {
-      mrb_raisef(mrb, E_TYPE_ERROR, "allocator undefined for %v", cv);
-    }
     mrb_raisef(mrb, E_TYPE_ERROR, "can't create instance of %v", cv);
   }
   o = (struct RObject*)mrb_obj_alloc(mrb, ttype, c);
diff --git a/src/gc.c b/src/gc.c
index 9eb84311b..39f63c62b 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -476,7 +476,6 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
     }
     tt = MRB_INSTANCE_TT(cls);
     if (tt != MRB_TT_FALSE &&
-        tt != MRB_TT_UNDEF &&
         ttype != MRB_TT_SCLASS &&
         ttype != MRB_TT_ICLASS &&
         ttype != MRB_TT_ENV &&
diff --git a/src/numeric.c b/src/numeric.c
index 5b098c8b8..743cdb3ae 100644
--- a/src/numeric.c
+++ b/src/numeric.c
@@ -2227,7 +2227,8 @@ mrb_init_numeric(mrb_state *mrb)
 
   /* Integer Class */
   mrb->integer_class = integer = mrb_define_class_id(mrb, MRB_SYM(Integer),  numeric);     /* 15.2.8 */
-  MRB_SET_INSTANCE_TT(integer, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(integer, MRB_TT_INTEGER);
+  MRB_UNDEF_ALLOCATOR(integer);
   mrb_undef_class_method_id(mrb, integer, MRB_SYM(new));
   mrb_define_method_id(mrb, integer, MRB_OPSYM(pow),    int_pow,         MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, integer, MRB_OPSYM(cmp),    num_cmp,         MRB_ARGS_REQ(1)); /* 15.2.8.3.1  */
@@ -2272,7 +2273,8 @@ mrb_init_numeric(mrb_state *mrb)
 #ifndef MRB_NO_FLOAT
   /* Float Class */
   mrb->float_class = fl = mrb_define_class_id(mrb, MRB_SYM(Float), numeric);               /* 15.2.9 */
-  MRB_SET_INSTANCE_TT(fl, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(fl, MRB_TT_FLOAT);
+  MRB_UNDEF_ALLOCATOR(fl);
   mrb_undef_class_method(mrb,  fl, "new");
   mrb_define_method_id(mrb, fl,      MRB_OPSYM(pow),     flo_pow,        MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, fl,      MRB_OPSYM(div),     flo_div,        MRB_ARGS_REQ(1)); /* 15.2.9.3.6 */
diff --git a/src/proc.c b/src/proc.c
index 978813f76..6aca2b84e 100644
--- a/src/proc.c
+++ b/src/proc.c
@@ -490,9 +490,8 @@ mrb_init_proc(mrb_state *mrb)
   mrb_method_t m;
   struct RClass *pc = mrb->proc_class = mrb_define_class_id(mrb, MRB_SYM(Proc), mrb->object_class); /* 15.2.17 */
 
-  MRB_SET_INSTANCE_TT(mrb->proc_class, MRB_TT_PROC);
-
-  MRB_SET_INSTANCE_TT(pc, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(pc, MRB_TT_PROC);
+  MRB_UNDEF_ALLOCATOR(pc);
   mrb_define_class_method_id(mrb, pc, MRB_SYM(new), mrb_proc_s_new, MRB_ARGS_NONE()|MRB_ARGS_BLOCK());
   mrb_define_method_id(mrb, pc, MRB_SYM(initialize_copy), mrb_proc_init_copy, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, pc, MRB_SYM(arity), proc_arity, MRB_ARGS_NONE()); /* 15.2.17.4.2 */