diff --git a/canonical-binary.md b/canonical-binary.md
index b592e3e..7d9b0fd 100644
--- a/canonical-binary.md
+++ b/canonical-binary.md
@@ -26,6 +26,8 @@ Annotations MUST NOT be present.
 Placeholders MUST NOT be used.
 Where possible, fixed-length ("format A") MUST be used in preference
 to variable-length ("format B") formats.
+Lengths in variable-length encodings (base 128 varints) MUST be as
+short as possible.
 
 **Signed integers.**
 When a `SignedInteger` *n* is greater than or equal to -3 and less
diff --git a/preserves.md b/preserves.md
index cc63a01..82b1684 100644
--- a/preserves.md
+++ b/preserves.md
@@ -516,6 +516,10 @@ The following table illustrates varint-encoding.
 | 300 | `0000010 0101100` | 172 2 |
 | 1000000000 | `0000011 1011100 1101011 0010100 0000000` | 128 148 235 220 3 |
 
+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.
+
 #### Streaming data of unknown length (format C).
 
 A `Repr` where the length of the `Value` to be encoded is variable and