diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index 416457d..ca94d72 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -4017,7 +4017,7 @@
 	0,					/* tp_setattro */
 	&string_as_buffer,			/* tp_as_buffer */
 	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_CHECKTYPES |
-		Py_TPFLAGS_BASETYPE,		/* tp_flags */
+		Py_TPFLAGS_BASETYPE | Py_TPFLAGS_STRING_SUBCLASS,		/* tp_flags */
 	string_doc,				/* tp_doc */
 	0,					/* tp_traverse */
 	0,					/* tp_clear */