diff --git a/src/kuroko/object.h b/src/kuroko/object.h
index a83890b..3c1354d 100644
--- a/src/kuroko/object.h
+++ b/src/kuroko/object.h
@@ -61,6 +61,7 @@ typedef struct KrkObj {
 #define KRK_OBJ_FLAGS_FUNCTION_IS_STATIC_METHOD    0x0002
 #define KRK_OBJ_FLAGS_FUNCTION_IS_DYNAMIC_PROPERTY 0x0004
 
+#define KRK_OBJ_FLAGS_NO_INHERIT    0x0200
 #define KRK_OBJ_FLAGS_SECOND_CHANCE 0x0100
 #define KRK_OBJ_FLAGS_IS_MARKED     0x0010
 #define KRK_OBJ_FLAGS_IN_REPR       0x0020
diff --git a/src/obj_base.c b/src/obj_base.c
index e2196fa..baa4540 100644
--- a/src/obj_base.c
+++ b/src/obj_base.c
@@ -79,7 +79,7 @@ static KrkValue _class_subclasses(int argc, const KrkValue argv[], int hasKw) {
 _noexport
 void _createAndBind_type(void) {
 	ADD_BASE_CLASS(vm.baseClasses->typeClass, "type", vm.baseClasses->objectClass);
-	vm.baseClasses->typeClass->allocSize = 0;
+	vm.baseClasses->typeClass->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	krk_defineNative(&vm.baseClasses->typeClass->methods, "__base__", krk_baseOfClass)->obj.flags = KRK_OBJ_FLAGS_FUNCTION_IS_DYNAMIC_PROPERTY;
 	krk_defineNative(&vm.baseClasses->typeClass->methods, "__file__", krk_fileOfClass)->obj.flags = KRK_OBJ_FLAGS_FUNCTION_IS_DYNAMIC_PROPERTY;
 	krk_defineNative(&vm.baseClasses->typeClass->methods, "__doc__", krk_docOfClass)  ->obj.flags = KRK_OBJ_FLAGS_FUNCTION_IS_DYNAMIC_PROPERTY;
diff --git a/src/obj_bytes.c b/src/obj_bytes.c
index 7d19186..2db69d1 100644
--- a/src/obj_bytes.c
+++ b/src/obj_bytes.c
@@ -379,7 +379,7 @@ KRK_METHOD(bytearray,__iter__,{
 _noexport
 void _createAndBind_bytesClass(void) {
 	KrkClass * bytes = ADD_BASE_CLASS(vm.baseClasses->bytesClass, "bytes", vm.baseClasses->objectClass);
-	bytes->allocSize = 0; /* cannot subclass */
+	bytes->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	KRK_DOC(BIND_METHOD(bytes,__init__),
 		"@brief An array of bytes.\n"
 		"@arguments iter=None\n\n"
diff --git a/src/obj_function.c b/src/obj_function.c
index ce2c0f8..60324c9 100644
--- a/src/obj_function.c
+++ b/src/obj_function.c
@@ -314,7 +314,7 @@ KRK_FUNC(classmethod,{
 _noexport
 void _createAndBind_functionClass(void) {
 	KrkClass * codeobject = ADD_BASE_CLASS(vm.baseClasses->codeobjectClass, "codeobject", vm.baseClasses->objectClass);
-	codeobject->allocSize = 0; /* cannot subclass */
+	codeobject->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(codeobject,__init__);
 	BIND_METHOD(codeobject,__str__);
 	BIND_METHOD(codeobject,_ip_to_line);
@@ -325,7 +325,7 @@ void _createAndBind_functionClass(void) {
 	krk_finalizeClass(codeobject);
 
 	KrkClass * function = ADD_BASE_CLASS(vm.baseClasses->functionClass, "function", vm.baseClasses->objectClass);
-	function->allocSize = 0; /* cannot subclass */
+	function->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(function,__init__);
 	BIND_METHOD(function,__str__);
 	BIND_METHOD(function,_ip_to_line);
@@ -341,7 +341,7 @@ void _createAndBind_functionClass(void) {
 	krk_finalizeClass(function);
 
 	KrkClass * method = ADD_BASE_CLASS(vm.baseClasses->methodClass, "method", vm.baseClasses->objectClass);
-	method->allocSize = 0; /* cannot subclass */
+	method->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(method,__str__);
 	BIND_METHOD(method,_ip_to_line);
 	BIND_METHOD(method,__init__);
diff --git a/src/obj_numeric.c b/src/obj_numeric.c
index cb3a5eb..ae6805e 100644
--- a/src/obj_numeric.c
+++ b/src/obj_numeric.c
@@ -136,7 +136,7 @@ KRK_METHOD(NotImplementedType,__hash__,{
 _noexport
 void _createAndBind_numericClasses(void) {
 	KrkClass * _int = ADD_BASE_CLASS(vm.baseClasses->intClass, "int", vm.baseClasses->objectClass);
-	_int->allocSize = 0; /* cannot subclass */
+	_int->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(int,__init__);
 	BIND_METHOD(int,__str__);
 	BIND_METHOD(int,__int__);
@@ -149,7 +149,7 @@ void _createAndBind_numericClasses(void) {
 	KRK_DOC(_int, "Convert a number or string type to an integer representation.");
 
 	KrkClass * _float = ADD_BASE_CLASS(vm.baseClasses->floatClass, "float", vm.baseClasses->objectClass);
-	_float->allocSize = 0; /* cannot subclass */
+	_float->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(float,__init__);
 	BIND_METHOD(float,__int__);
 	BIND_METHOD(float,__float__);
@@ -161,7 +161,7 @@ void _createAndBind_numericClasses(void) {
 	KRK_DOC(_float, "Convert a number or string type to a float representation.");
 
 	KrkClass * _bool = ADD_BASE_CLASS(vm.baseClasses->boolClass, "bool", vm.baseClasses->intClass);
-	_bool->allocSize = 0; /* cannot subclass */
+	_bool->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(bool,__init__);
 	BIND_METHOD(bool,__str__);
 	krk_defineNative(&_bool->methods, "__repr__", FUNC_NAME(bool,__str__));
@@ -169,14 +169,14 @@ void _createAndBind_numericClasses(void) {
 	KRK_DOC(_bool, "Returns False if the argument is 'falsey', otherwise True.");
 
 	KrkClass * _NoneType = ADD_BASE_CLASS(vm.baseClasses->noneTypeClass, "NoneType", vm.baseClasses->objectClass);
-	_NoneType->allocSize = 0; /* cannot subclass */
+	_NoneType->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(NoneType, __str__);
 	BIND_METHOD(NoneType, __hash__);
 	krk_defineNative(&_NoneType->methods, "__repr__", FUNC_NAME(NoneType,__str__));
 	krk_finalizeClass(_NoneType);
 
 	KrkClass * _NotImplementedType = ADD_BASE_CLASS(vm.baseClasses->notImplClass, "NotImplementedType", vm.baseClasses->objectClass);
-	_NotImplementedType->allocSize = 0; /* cannot subclass */
+	_NotImplementedType->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(NotImplementedType, __str__);
 	BIND_METHOD(NotImplementedType, __hash__);
 	krk_defineNative(&_NotImplementedType->methods, "__repr__", FUNC_NAME(NotImplementedType,__str__));
diff --git a/src/obj_str.c b/src/obj_str.c
index 0c3413a..e008766 100644
--- a/src/obj_str.c
+++ b/src/obj_str.c
@@ -889,7 +889,7 @@ _corrupt:
 _noexport
 void _createAndBind_strClass(void) {
 	KrkClass * str = ADD_BASE_CLASS(vm.baseClasses->strClass, "str", vm.baseClasses->objectClass);
-	str->allocSize = 0; /* cannot subclass */
+	str->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(str,__init__);
 	BIND_METHOD(str,__iter__);
 	BIND_METHOD(str,__ord__);
diff --git a/src/obj_tuple.c b/src/obj_tuple.c
index 3ae531e..f34362b 100644
--- a/src/obj_tuple.c
+++ b/src/obj_tuple.c
@@ -203,7 +203,7 @@ _unhashable:
 _noexport
 void _createAndBind_tupleClass(void) {
 	KrkClass * tuple = ADD_BASE_CLASS(vm.baseClasses->tupleClass, "tuple", vm.baseClasses->objectClass);
-	tuple->allocSize = 0; /* cannot subclass */
+	tuple->obj.flags |= KRK_OBJ_FLAGS_NO_INHERIT;
 	BIND_METHOD(tuple,__repr__);
 	BIND_METHOD(tuple,__getitem__);
 	BIND_METHOD(tuple,__len__);
diff --git a/src/object.c b/src/object.c
index e0c81a6..348e522 100644
--- a/src/object.c
+++ b/src/object.c
@@ -324,8 +324,7 @@ KrkClass * krk_newClass(KrkString * name, KrkClass * baseClass) {
 }
 
 KrkInstance * krk_newInstance(KrkClass * _class) {
-	size_t allocSize = _class->allocSize ? _class->allocSize : sizeof(KrkInstance);
-	KrkInstance * instance = (KrkInstance*)allocateObject(allocSize, KRK_OBJ_INSTANCE);
+	KrkInstance * instance = (KrkInstance*)allocateObject(_class->allocSize, KRK_OBJ_INSTANCE);
 	instance->_class = _class;
 	krk_initTable(&instance->fields);
 	return instance;
diff --git a/src/vm.c b/src/vm.c
index a1e7a13..70ecce4 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -2590,7 +2590,7 @@ _finishReturn: (void)0;
 						krk_typeName(superclass));
 					goto _finishException;
 				}
-				if (AS_CLASS(superclass)->allocSize == 0) {
+				if (AS_OBJECT(superclass)->flags & KRK_OBJ_FLAGS_NO_INHERIT) {
 					krk_runtimeError(vm.exceptions->typeError, "'%s' can not be subclassed",
 						AS_CLASS(superclass)->name->chars);
 					goto _finishException;