diff --git a/Python/marshal.c b/Python/marshal.c
index 8d441a4..04a8dc5 100644
--- a/Python/marshal.c
+++ b/Python/marshal.c
@@ -813,7 +813,7 @@
     if (ob == NULL)
         return NULL;
 
-    Py_SIZE(ob) = n > 0 ? size : -size;
+    Py_SET_SIZE(ob, n > 0 ? size : -size);
 
     for (i = 0; i < size-1; i++) {
         d = 0;