diff --git a/py/objint.c b/py/objint.c
index d1d99a2..34447b3 100644
--- a/py/objint.c
+++ b/py/objint.c
@@ -129,7 +129,7 @@
 //
 // The resulting formatted string will be returned from this function and the
 // formatted size will be in *fmt_size.
-char *mp_obj_int_formatted(char **buf, int *buf_size, int *fmt_size, mp_obj_t self_in,
+char *mp_obj_int_formatted(char **buf, int *buf_size, int *fmt_size, mp_const_obj_t self_in,
                            int base, const char *prefix, char base_char, char comma) {
     fmt_int_t num;
     if (MP_OBJ_IS_SMALL_INT(self_in)) {