diff --git a/src/test/test-calendarspec.c b/src/test/test-calendarspec.c
index 152ce879f8..c62e6860cf 100644
--- a/src/test/test-calendarspec.c
+++ b/src/test/test-calendarspec.c
@@ -44,7 +44,7 @@ static void _test_next(int line, const char *input, const char *new_tz, usec_t a
         if (old_tz)
                 old_tz = strdupa(old_tz);
 
-        if (new_tz)
+        if (!isempty(new_tz))
                 new_tz = strjoina(":", new_tz);
 
         assert_se(set_unset_env("TZ", new_tz, true) == 0);