diff --git a/src/test/test-time-util.c b/src/test/test-time-util.c
index e0386d4ec5..a7b2467452 100644
--- a/src/test/test-time-util.c
+++ b/src/test/test-time-util.c
@@ -663,6 +663,11 @@ static bool timezone_equal(usec_t today, usec_t target) {
 static void test_parse_timestamp_impl(const char *tz) {
         usec_t today, now_usec;
 
+        /* Invalid: Ensure that systemctl reboot --when=show and --when=cancel
+         * will not result in ambiguities */
+        assert_se(parse_timestamp("show", NULL) == -EINVAL);
+        assert_se(parse_timestamp("cancel", NULL) == -EINVAL);
+
         /* UTC */
         test_parse_timestamp_one("Thu 1970-01-01 00:01 UTC", 0, USEC_PER_MINUTE);
         test_parse_timestamp_one("Thu 1970-01-01 00:00:01 UTC", 0, USEC_PER_SEC);