diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 006df8d1f0..9e5ddef0c1 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -1003,8 +1003,11 @@ PyObject *
 PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)
 {
     PyObject *obj;
-    const size_t size = _PyObject_VAR_SIZE(type, nitems+1);
-    /* note that we need to add one, for the sentinel */
+    size_t extra = 0;
+    if (type->tp_flags & Py_TPFLAGS_TYPE_SUBCLASS) {
+        extra = 1; // need a sentinel at end for types
+    }
+    const size_t size = _PyObject_VAR_SIZE(type, nitems+extra);
 
     if (PyType_IS_GC(type))
         obj = _PyObject_GC_Malloc(size);