diff --git a/Blog-Sharing-Client.md b/Blog-Sharing-Client.md
index a4cfd84..5290556 100644
--- a/Blog-Sharing-Client.md
+++ b/Blog-Sharing-Client.md
@@ -50,7 +50,7 @@ Aborting from any state returns the session to the START state.
 
 ### Validity policy
 
-* A message is valid if it is well-formed.
+* A message is valid if it is well-formed and its previous message (if any) is a valid message in the same session.
 
 ### Storage policy