diff --git a/doc/specs/HeaderFormat.md b/doc/specs/HeaderFormat.md
index 9b74159..bcd1e2d 100644
--- a/doc/specs/HeaderFormat.md
+++ b/doc/specs/HeaderFormat.md
@@ -78,5 +78,6 @@
 
     INFO_KEYVALUE 0x01 - varint32 number of headers.
                        - key/value pairs of varstrings (varint16 length plus
-                         no-trailing-null string).
+                         no-trailing-null string). Implementations MUST NOT
+                         alter either key or value in any way.