diff --git a/Include/stringobject.h b/Include/stringobject.h
index 1d932ff..c6ae1a8 100644
--- a/Include/stringobject.h
+++ b/Include/stringobject.h
@@ -85,7 +85,7 @@
 
 /* Macro, trading safety for speed */
 #define PyString_AS_STRING(op) (assert(PyString_Check(op)),(((PyStringObject *)(op))->ob_sval))
-#define PyString_GET_SIZE(op)  (assert(PyString_Check(op)),(((PyStringObject *)(op))->ob_size))
+#define PyString_GET_SIZE(op)  (assert(PyString_Check(op)),Py_Size(op))
 
 /* _PyString_Join(sep, x) is like sep.join(x).  sep must be PyStringObject*,
    x must be an iterable object. */