diff --git a/draft-ietf-rats-msg-wrap.md b/draft-ietf-rats-msg-wrap.md
index efc58c4..bdad029 100644
--- a/draft-ietf-rats-msg-wrap.md
+++ b/draft-ietf-rats-msg-wrap.md
@@ -271,6 +271,8 @@ The CMW collection ({{fig-cddl-collection}}) is defined as a CBOR map or JSON ob
 The position of a `cmw` entry in the `cmw-collection` is not significant.
 Labels can be strings (or integers in the CBOR serialization) that serve as a mnemonic for different conceptual messages in the collection.
 
+A collection MUST have at least one CMW entry.
+
 The `"__cmwc_t"` key is reserved for associating an optional type to the overall collection and MUST NOT be used for a label.
 The collection type is either a Uniform Resource Identifier (URI) or an object identifier (OID).
 The OID is always absolute and never relative.