diff --git a/block/qcow.c b/block/qcow.c
index 8546fe5bb7..d7c385a91d 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -345,8 +345,8 @@ static int qcow_reopen_prepare(BDRVReopenState *state,
  *
  * 0 to not allocate.
  *
- * 1 to allocate a normal cluster (for sector indexes 'n_start' to
- * 'n_end')
+ * 1 to allocate a normal cluster (for sector-aligned byte offsets 'n_start'
+ * to 'n_end' within the cluster)
  *
  * 2 to allocate a compressed cluster of size
  * 'compressed_size'. 'compressed_size' must be > 0 and <
@@ -440,9 +440,10 @@ static int get_cluster_offset(BlockDriverState *bs,
         if (!allocate)
             return 0;
         BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC);
+        assert(QEMU_IS_ALIGNED(n_start | n_end, BDRV_SECTOR_SIZE));
         /* allocate a new cluster */
         if ((cluster_offset & QCOW_OFLAG_COMPRESSED) &&
-            (n_end - n_start) < s->cluster_sectors) {
+            (n_end - n_start) < s->cluster_size) {
             /* if the cluster is already compressed, we must
                decompress it in the case it is not completely
                overwritten */
@@ -480,16 +481,15 @@ static int get_cluster_offset(BlockDriverState *bs,
                 /* if encrypted, we must initialize the cluster
                    content which won't be written */
                 if (bs->encrypted &&
-                    (n_end - n_start) < s->cluster_sectors) {
-                    uint64_t start_sect;
+                    (n_end - n_start) < s->cluster_size) {
+                    uint64_t start_offset;
                     assert(s->crypto);
-                    start_sect = (offset & ~(s->cluster_size - 1)) >> 9;
-                    for(i = 0; i < s->cluster_sectors; i++) {
+                    start_offset = offset & ~(s->cluster_size - 1);
+                    for (i = 0; i < s->cluster_size; i += BDRV_SECTOR_SIZE) {
                         if (i < n_start || i >= n_end) {
-                            memset(s->cluster_data, 0x00, 512);
+                            memset(s->cluster_data, 0x00, BDRV_SECTOR_SIZE);
                             if (qcrypto_block_encrypt(s->crypto,
-                                                      (start_sect + i) *
-                                                      BDRV_SECTOR_SIZE,
+                                                      start_offset + i,
                                                       s->cluster_data,
                                                       BDRV_SECTOR_SIZE,
                                                       NULL) < 0) {
@@ -497,8 +497,9 @@ static int get_cluster_offset(BlockDriverState *bs,
                             }
                             BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
                             ret = bdrv_pwrite(bs->file,
-                                              cluster_offset + i * 512,
-                                              s->cluster_data, 512);
+                                              cluster_offset + i,
+                                              s->cluster_data,
+                                              BDRV_SECTOR_SIZE);
                             if (ret < 0) {
                                 return ret;
                             }
@@ -758,8 +759,8 @@ static coroutine_fn int qcow_co_writev(BlockDriverState *bs, int64_t sector_num,
             n = nb_sectors;
         }
         ret = get_cluster_offset(bs, sector_num << 9, 1, 0,
-                                 index_in_cluster,
-                                 index_in_cluster + n, &cluster_offset);
+                                 index_in_cluster << 9,
+                                 (index_in_cluster + n) << 9, &cluster_offset);
         if (ret < 0) {
             break;
         }