diff --git a/py/runtime.c b/py/runtime.c
index 409d1d8262da28e7f2035c43fdece99c8dac42bc..b982ee32d025d50087ffe841677430d0dbd10913 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -553,6 +553,8 @@ mp_obj_t rt_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
             if (fit_small_int(lhs_val)) {
                 return MP_OBJ_NEW_SMALL_INT(lhs_val);
             }
+            // TODO: return long int
+            assert(0);
         } else if (MP_OBJ_IS_TYPE(rhs, &float_type)) {
             return mp_obj_float_binary_op(op, lhs_val, rhs);
         } else if (MP_OBJ_IS_TYPE(rhs, &complex_type)) {