diff --git a/Misc/NEWS b/Misc/NEWS
index 50f6aaf..e26b6ca 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,9 @@
 Core and Builtins
 -----------------
 
+- Issue #8651: Fix "z#" format of PyArg_Parse*() function: the size was not
+  written if PY_SSIZE_T_CLEAN is defined.
+
 - Issue #9756: When calling a method descriptor or a slot wrapper descriptor,
   the check of the object type doesn't read the __class__ attribute anymore.
   Fix a crash if a class override its __class__ attribute (e.g. a proxy of the
diff --git a/Python/getargs.c b/Python/getargs.c
index 7c3e9fa..02351ed 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -984,10 +984,11 @@
             if (*format == '#') {
                 FETCH_SIZE;
                 assert(0); /* XXX redundant with if-case */
-                if (arg == Py_None)
-                    *q = 0;
-                else
-                    *q = PyString_Size(arg);
+                if (arg == Py_None) {
+                    STORE_SIZE(0);
+                } else {
+                    STORE_SIZE(PyString_Size(arg));
+                }
                 format++;
             }
             else if (*p != NULL &&