diff --git a/preserves.md b/preserves.md
index 82b1684..29d7a29 100644
--- a/preserves.md
+++ b/preserves.md
@@ -518,7 +518,10 @@ The following table illustrates varint-encoding.
 
 It is an error for a varint-encoded `m` in a `Repr` to be anything
 other than the unique shortest encoding for that `m`. That is, a
-varint-encoding of `m` *MUST NOT* end in `0` unless `m`=0.
+varint-encoding of `m` *MUST NOT* end in `0` unless `m`=0. However,
+the `varint(m)` encoding of a length *MUST NOT* be used when `m`<15,
+meaning that a `Repr` *MUST NOT* contain any varint-encoding with
+final byte `0`.
 
 #### Streaming data of unknown length (format C).