diff --git a/esp8266/modutime.c b/esp8266/modutime.c
index 2adb6c563b..117720e6f6 100644
--- a/esp8266/modutime.c
+++ b/esp8266/modutime.c
@@ -84,7 +84,7 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(time_localtime_obj, 0, 1, time_localtime);
 /// which expresses a time as per localtime. It returns an integer which is
 /// the number of seconds since Jan 1, 2000.
 STATIC mp_obj_t time_mktime(mp_obj_t tuple) {
-    mp_uint_t len;
+    size_t len;
     mp_obj_t *elem;
     mp_obj_get_array(tuple, &len, &elem);