Update draft-ietf-moq-transport.md

Author: englishm

draft-ietf-moq-transport.md

@@ -1443,7 +1443,7 @@ Fields common to all Fetch Types:
 is a variable length integer that MUST be unique and monotonically increasing
 within  a session. For a Standalone Fetch a new Subscribe ID MUST be used. For
 a Joining Fetch, the Subscribe ID MUST correspond to a Subscribe which has already
-been sent.
+been sent. If a publisher receives a Joining Fetch with a Subscribe ID that does not correspond to an existing Subscribe, it MUST close the session with an Invalid Subscribe ID error.
 
 * Subscriber Priority: Specifies the priority of a fetch request relative to
 other subscriptions or fetches in the same session. Lower numbers get higher