diff --git a/block/bio.c b/block/bio.c
index 194d28cdc642..2e5d881423b8 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -156,7 +156,7 @@ static void bio_put_slab(struct bio_set *bs)
 
 unsigned int bvec_nr_vecs(unsigned short idx)
 {
-	return bvec_slabs[idx].nr_vecs;
+	return bvec_slabs[--idx].nr_vecs;
 }
 
 void bvec_free(mempool_t *pool, struct bio_vec *bv, unsigned int idx)