diff --git a/py/objint_longlong.c b/py/objint_longlong.c
index f10e464..b051cfb 100644
--- a/py/objint_longlong.c
+++ b/py/objint_longlong.c
@@ -53,7 +53,7 @@
 const mp_obj_int_t mp_maxsize_obj = {{&mp_type_int}, MP_SSIZE_MAX};
 #endif
 
-void mp_obj_int_to_bytes_impl(mp_obj_t self_in, bool big_endian, mp_uint_t len, byte *buf) {
+void mp_obj_int_to_bytes_impl(mp_obj_t self_in, bool big_endian, size_t len, byte *buf) {
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_int));
     mp_obj_int_t *self = self_in;
     long long val = self->val;