diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 8882a1d1d4..6938a75aa5 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -930,7 +930,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
     virtio_stw_p(vdev, &blkcfg.geometry.cylinders, conf->cyls);
     virtio_stl_p(vdev, &blkcfg.blk_size, blk_size);
     virtio_stw_p(vdev, &blkcfg.min_io_size, conf->min_io_size / blk_size);
-    virtio_stw_p(vdev, &blkcfg.opt_io_size, conf->opt_io_size / blk_size);
+    virtio_stl_p(vdev, &blkcfg.opt_io_size, conf->opt_io_size / blk_size);
     blkcfg.geometry.heads = conf->heads;
     /*
      * We must ensure that the block device capacity is a multiple of