diff --git a/extmod/utime_mphal.c b/extmod/utime_mphal.c
index 3ecdc94469..4a03bf94b1 100644
--- a/extmod/utime_mphal.c
+++ b/extmod/utime_mphal.c
@@ -78,11 +78,11 @@ STATIC mp_obj_t time_ticks_cpu(void) {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(mp_utime_ticks_cpu_obj, time_ticks_cpu);
 
-STATIC mp_obj_t time_ticks_diff(mp_obj_t start_in, mp_obj_t end_in) {
+STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
     // we assume that the arguments come from ticks_xx so are small ints
     uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
     uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
-    return MP_OBJ_NEW_SMALL_INT((end - start) & MP_SMALL_INT_POSITIVE_MASK);
+    return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);