diff --git a/esp8266/modutime.c b/esp8266/modutime.c
index 2adb6c5..117720e 100644
--- a/esp8266/modutime.c
+++ b/esp8266/modutime.c
@@ -84,7 +84,7 @@
 /// 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);