diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 63dbc43..bf7e4ac 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -412,7 +412,7 @@
     if (PyTuple_GET_SIZE(args) > 0) {
         if (numfields != PyTuple_GET_SIZE(args)) {
             PyErr_Format(PyExc_TypeError, "%.400s constructor takes %s"
-                         "%" PY_FORMAT_SIZE_T "d positional argument%s",
+                         "%zd positional argument%s",
                          Py_TYPE(self)->tp_name,
                          numfields == 0 ? "" : "either 0 or ",
                          numfields, numfields == 1 ? "" : "s");