diff --git a/Python/ast.c b/Python/ast.c
index 821a5ad..5845076 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -3101,8 +3101,9 @@
                     Py_DECREF(u);
                     return NULL;
                 }
-                r = PyString_AsString(w);
-                rn = PyString_Size(w);
+                assert(PyBytes_Check(w));
+                r = PyBytes_AsString(w);
+                rn = PyBytes_Size(w);
                 assert(rn % 2 == 0);
                 for (i = 0; i < rn; i += 2) {
                     sprintf(p, "\\u%02x%02x",