diff --git a/src/lib-sieve/cmd-redirect.c b/src/lib-sieve/cmd-redirect.c
index c64197d8b1ba57efbbb6a4c2423d15c53bed57bc..0647ce11d123e523bf2035b53dc5d2783d75d38e 100644
--- a/src/lib-sieve/cmd-redirect.c
+++ b/src/lib-sieve/cmd-redirect.c
@@ -539,6 +539,7 @@ act_redirect_commit(const struct sieve_action *action,
 	ret = act_redirect_get_duplicate_id(ctx, aenv, msg_id, &dupeid);
 	if (ret != SIEVE_EXEC_OK)
 		return ret;
+	i_assert(dupeid != NULL);
 
 	/* Check whether we've seen this message before */
 	if (sieve_action_duplicate_check(senv, dupeid, strlen(dupeid))) {