diff --git a/src/lib-sieve/sieve-stringlist.c b/src/lib-sieve/sieve-stringlist.c
index 9e226e5024e86a814a14e849af84de7b8aca52d0..17b7d5ce140187bdcf46957d44080e287e30a8a3 100644
--- a/src/lib-sieve/sieve-stringlist.c
+++ b/src/lib-sieve/sieve-stringlist.c
@@ -218,6 +218,7 @@ static int sieve_index_stringlist_next_item
 		index = strlist->index;
 	}
 
+	i_assert(index > 0);
 	while ( index > 0 ) {
 		if ( (ret=sieve_stringlist_next_item(strlist->source, str_r)) <= 0 ) {
 			if (ret < 0)