diff --git a/sys/cddl/boot/zfs/zfsimpl.h b/sys/cddl/boot/zfs/zfsimpl.h
--- a/sys/cddl/boot/zfs/zfsimpl.h
+++ b/sys/cddl/boot/zfs/zfsimpl.h
@@ -536,6 +536,12 @@
 	offsetof(vdev_label_t, vl_uberblock[(n) << VDEV_UBERBLOCK_SHIFT(vd)])
 #define	VDEV_UBERBLOCK_SIZE(vd)		(1ULL << VDEV_UBERBLOCK_SHIFT(vd))
 
+#define	ASHIFT_UBERBLOCK_SHIFT(ashift)  \
+	MIN(MAX(ashift, UBERBLOCK_SHIFT), \
+	MAX_UBERBLOCK_SHIFT)
+#define	ASHIFT_UBERBLOCK_SIZE(ashift) \
+	(1ULL << ASHIFT_UBERBLOCK_SHIFT(ashift))
+
 typedef struct vdev_phys {
 	char		vp_nvlist[VDEV_PHYS_SIZE - sizeof (zio_eck_t)];
 	zio_eck_t	vp_zbt;
diff --git a/usr.sbin/makefs/zfs.c b/usr.sbin/makefs/zfs.c
--- a/usr.sbin/makefs/zfs.c
+++ b/usr.sbin/makefs/zfs.c
@@ -596,7 +596,7 @@
 	 * checksum is calculated in vdev_label_write().
 	 */
 	for (size_t uoff = 0; uoff < sizeof(label->vl_uberblock);
-	    uoff += (1 << zfs->ashift)) {
+	    uoff += ASHIFT_UBERBLOCK_SIZE(zfs->ashift)) {
 		ub = (uberblock_t *)(&label->vl_uberblock[0] + uoff);
 		ub->ub_magic = UBERBLOCK_MAGIC;
 		ub->ub_version = SPA_VERSION;
diff --git a/usr.sbin/makefs/zfs/vdev.c b/usr.sbin/makefs/zfs/vdev.c
--- a/usr.sbin/makefs/zfs/vdev.c
+++ b/usr.sbin/makefs/zfs/vdev.c
@@ -200,7 +200,7 @@
 	 * per sector; for example, with an ashift of 12 we end up with
 	 * 128KB/4KB=32 copies of the uberblock in the ring.
 	 */
-	blksz = 1 << zfs->ashift;
+	blksz = ASHIFT_UBERBLOCK_SIZE(zfs->ashift);
 	assert(sizeof(label->vl_uberblock) % blksz == 0);
 	for (size_t roff = 0; roff < sizeof(label->vl_uberblock);
 	    roff += blksz) {