diff --git a/uts/common/fs/zfs/vdev_disk.c b/uts/common/fs/zfs/vdev_disk.c
index ed4a8b773bf4..9ed59b232e1d 100644
--- a/uts/common/fs/zfs/vdev_disk.c
+++ b/uts/common/fs/zfs/vdev_disk.c
@@ -796,6 +796,8 @@ vdev_disk_io_start(zio_t *zio)
 		return;
 	}
 
+	ASSERT(zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE);
+
 	vb = kmem_alloc(sizeof (vdev_buf_t), KM_SLEEP);
 
 	vb->vb_io = zio;
diff --git a/uts/common/fs/zfs/vdev_file.c b/uts/common/fs/zfs/vdev_file.c
index 5dfc331d20a4..d5bad45f7f75 100644
--- a/uts/common/fs/zfs/vdev_file.c
+++ b/uts/common/fs/zfs/vdev_file.c
@@ -211,6 +211,8 @@ vdev_file_io_start(zio_t *zio)
 		return;
 	}
 
+	ASSERT(zio->io_type == ZIO_TYPE_READ || zio->io_type == ZIO_TYPE_WRITE);
+
 	vb = kmem_alloc(sizeof (vdev_buf_t), KM_SLEEP);
 
 	vb->vb_io = zio;