diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index 29e000b..6bb2200 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -423,7 +423,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(buffered);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buffer)
         res += self->buffer_size;
     return PyLong_FromSsize_t(res);