diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index c6dcb58f600..bff74d92770 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -12254,7 +12254,7 @@ PyUnicode_Format(PyObject *format, PyObject *args)
             case 'c':
                 pbuf = formatbuf;
                 kind = PyUnicode_4BYTE_KIND;
-                len = formatchar(pbuf, sizeof(formatbuf)/sizeof(Py_UNICODE), v);
+                len = formatchar(pbuf, Py_ARRAY_LENGTH(formatbuf), v);
                 if (len < 0)
                     goto onError;
                 break;