diff --git a/tools/vals.h b/tools/vals.h
index 098dc9e4e..addbea2f2 100644
--- a/tools/vals.h
+++ b/tools/vals.h
@@ -127,7 +127,7 @@ val(pextents_VAL, pextents_arg, "PExtents", "[+]Number[PERCENT]")
 val(nextents_VAL, nextents_arg, "NExtents", "[-]Number[PERCENT]")
 val(permission_VAL, permission_arg, "Permission", "rw|r")
 val(metadatatype_VAL, metadatatype_arg, "MetadataType", "lvm2")
-val(units_VAL, string_arg, "Units", "r|R|h|H|b|B|s|S|k|K|m|M|g|G|t|T|p|P|e|E")
+val(units_VAL, string_arg, "Units", "[Number]r|R|h|H|b|B|s|S|k|K|m|M|g|G|t|T|p|P|e|E")
 val(segtype_VAL, segtype_arg, "SegType", "linear|striped|snapshot|raid|mirror|thin|thin-pool|vdo|vdo-pool|cache|cache-pool|writecache")
 val(alloc_VAL, alloc_arg, "Alloc", "contiguous|cling|cling_by_tags|normal|anywhere|inherit")
 val(locktype_VAL, locktype_arg, "LockType", "sanlock|dlm|none")