diff --git a/Objects/structseq.c b/Objects/structseq.c
index c86fbe5..1865e24 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -47,7 +47,7 @@
         return NULL;
     /* Hack the size of the variable object, so invisible fields don't appear
      to Python code. */
-    Py_SIZE(obj) = VISIBLE_SIZE_TP(type);
+    Py_SET_SIZE(obj, VISIBLE_SIZE_TP(type));
     for (i = 0; i < size; i++)
         obj->ob_item[i] = NULL;