diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index d89b610..0ccf489 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -410,7 +410,7 @@
         }
         PyTuple_SET_ITEM(fnames, i, field);
     }
-    result = PyObject_CallFunction((PyObject*)&PyType_Type, "s(O){sOss}",
+    result = PyObject_CallFunction((PyObject*)&PyType_Type, "U(O){sOss}",
                     type, base, "_fields", fnames, "__module__", "_ast");
     Py_DECREF(fnames);
     return (PyTypeObject*)result;