diff --git a/t/unit-tests/t-reftable-readwrite.c b/t/unit-tests/t-reftable-readwrite.c
index 9a05dde9d6d39d..2ce56a052377fa 100644
--- a/t/unit-tests/t-reftable-readwrite.c
+++ b/t/unit-tests/t-reftable-readwrite.c
@@ -774,6 +774,11 @@ static void t_write_key_order(void)
 	check(!err);
 	err = reftable_writer_add_ref(w, &refs[1]);
 	check_int(err, ==, REFTABLE_API_ERROR);
+
+	refs[0].update_index = 2;
+	err = reftable_writer_add_ref(w, &refs[0]);
+	check_int(err, ==, REFTABLE_API_ERROR);
+
 	reftable_writer_close(w);
 	reftable_writer_free(w);
 	strbuf_release(&buf);