diff -r d489394a73de Modules/parsermodule.c
--- a/Modules/parsermodule.c	Tue Dec 17 15:12:46 2013 +0200
+++ b/Modules/parsermodule.c	Thu Dec 19 12:14:15 2013 +0100
@@ -785,7 +785,7 @@ build_node_children(PyObject *tuple, nod
             }
         }
         if (!ok) {
-            PyObject *err = Py_BuildValue("os", elem,
+            PyObject *err = Py_BuildValue("Os", elem,
                                           "Illegal node construct.");
             PyErr_SetObject(parser_error, err);
             Py_XDECREF(err);