diff --git a/Objects/listobject.c b/Objects/listobject.c
index dcd7b5e..05dddfc 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -804,6 +804,9 @@
             Py_RETURN_NONE;
         }
         m = Py_SIZE(self);
+        /* It should not be possible to allocate a list large enough to cause
+        an overflow on any relevant platform */
+        assert(m < PY_SSIZE_T_MAX - n);
         if (list_resize(self, m + n) < 0) {
             Py_DECREF(b);
             return NULL;