diff --git a/py/persistentcode.c b/py/persistentcode.c
index c8d3bb8a66..cc6e161f42 100644
--- a/py/persistentcode.c
+++ b/py/persistentcode.c
@@ -560,7 +560,7 @@ STATIC void save_obj(mp_print_t *print, mp_obj_t o) {
         } else {
             obj_type = 'b';
         }
-        mp_uint_t len;
+        size_t len;
         const char *str = mp_obj_str_get_data(o, &len);
         mp_print_bytes(print, &obj_type, 1);
         mp_print_uint(print, len);